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Polymer 54 (2013) 3350e3362

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Polymer

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Thermally modulated nanostructure of poly(ε-caprolactone)ePOSSmultiblock thermoplastic polyurethanes

Estefania Huitron-Rattinger a,b, Kazuki Ishida c, Angel Romo-Uribe b,**, Patrick T. Mather c,*aDepartamento de Ingeniería Química Metalúrgica, Facultad de Química, Universidad Nacional Autónoma de México, 04510 México D.F., Mexicob Laboratorio de Nanopolimeros y Coloides, Instituto de Ciencias Fisicas, Universidad Nacional Autónoma de México, Cuernavaca, Mor. 62210, Mexicoc Syracuse Biomaterials Institute and Department of Biomedical and Chemical Engineering, Syracuse University, Syracuse, NY 13244, United States

a r t i c l e i n f o

Article history:Received 8 January 2013Received in revised form3 April 2013Accepted 8 April 2013Available online 15 April 2013

Keywords:POSSThermoplastic polyurethaneCompetitive crystallization

* Corresponding author. Tel.: þ1 315 443 8760.** Corresponding author. Tel.: þ52 777 329 0880.

E-mail addresses: aromo-uribe@fis.unam.mx (Asyr.edu (P.T. Mather).

0032-3861/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.polymer.2013.04.015

a b s t r a c t

A series of multiblock polyurethanes with alternating sequence structures of a poly(ε-caprolactone) (PCL)segment of 2600 or 3600 g/mol and a polyhedral oligomeric silsesquioxane (POSS) segment with mul-tiple POSS moieties (TPU2.6k_1-x or TPU3.6k_1-x, respectively; the molar ratio of PCL:POSS is 1: x; x ¼ 2,3, or 4) were synthesized through two-step polymerization to assure quantitative conversion of re-actants. Differential scanning calorimetry and simultaneous wide- and small-angle X-ray scatteringmeasurements were performed to study the nanostructures of those samples. The multiblock andalternating sequence structures provided nano-confined environments for PCL and POSS domains, whichsignificantly suppressed crystallinity of the PCL phase, while nano-sized crystallites were formed in thePOSS phase. The samples in series TPU2.6k and TPU3.6k were also proven to display either lamellar,cubic, or cylindrical hexagonal phase-separated nanostructures depending on the molecular weight ofthe PCL segment, as well as the PCL/POSS ratio. It was also found that repeated thermal cycling under anitrogen atmosphere low enough in temperature not to alter molecular weight caused larger and moreordered PCL and POSS crystalline structure to form for the TPU3.6k series. Apparent reconfiguration ofthe PCL and POSS moieties along the backbone by exchange reactions associated with reversibility ofurethane bonds led to increases in PCL and POSS block lengths in the TPU chains. We envision an op-portunity of future research and applicability in the areas of tailored toughness and rigidity in biode-gradable polymer coatings, devices with enlarged data storage capacity, drug delivery systems and tissueengineering.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Multiblock polymers ranging from simple diblock and triblockcopolymers to more complicated multi-segmented polymers aremacromolecules composed of linear or nonlinear arrangements ofchemically distinct, phase-segregating segments [1]. In most casesthe covalent bonding between segmented blocks that are inher-ently incompatible with each other limits phase separation to thenano-scale and gives rise to a rich variety of well-defined self-assembled structures [2]. The selection of appropriate combina-tions of different blocks, and the design and modulation of the self-assembled nanostructures of multiblock copolymers, each plays amajor role in materials design for number of applications such as

. Romo-Uribe), ptmather@

All rights reserved.

thermoplastic elastomers [3], stimuli-responsive shape memorypolymers [4e7], information storage [8], and drug delivery [9e11].

Thermoplastic polyurethanes (TPUs), a focus of the presentwork, are segmented polymers comprising alternating sequencesof multi-components. Dihydroxy-terminated macromers (or “pol-yols”), and diisocyanates are used to synthesize linear TPU chains,yielding a “multiblock” architecture. The large number of possiblecombinations of polyols and diisocyanates and the facile synthesisenable preparation of TPUs targeting specific requirements such asdesired nano- and micro-structures, both in the solid and moltenstates. Order at the nanoscale can profoundly impact physicalproperties including thermal, mechanical, and optical properties. Inaddition, chain extenders, normally small molecule diols, arefrequently used during the TPU synthesis to form further higher-molecular-weight chains and even introduce other functionalities.

Polyhedral oligomeric silsesquioxane (POSS), another focus ofthe present work, is awell-defined nano-sizedmolecule (molecularweight ¼ w1000 g/mol) with an inorganic silica-like (SiO1.5) core

E. Huitron-Rattinger et al. / Polymer 54 (2013) 3350e3362 3351

surrounded by a shell of organic vertex groups. A wide variety ofthe organic vertex groups including not only simple alkyl hydro-carbon groups but reactive functional groups such as hydroxyl,carboxyl, amino, epoxy, acrylyl, and mercapto groups can beintroduced into POSS. Such groups serve to compatibilize POSSwith organic “hosts,” whether covalently attached or blended. Thereactive groups, in particular, enable covalent connections of POSSto the polymer chains, which generally improves dispersion ofPOSS in the polymer matrix. Thus POSS has been incorporated intovarious polymers via copolymerization [12], grafting [13e15], orblending [11,16e18] to prepare a diversity of polymernanocomposites.

POSS-containing polymer nanocomposites have been used formany applications such as electronics [19,20], medical engineering[21,22], biomaterials [23], and stimuli-responsive shape memory[24,25]. Recently, Wu and Mather [26] have reviewed the diverseand significant research on POSS-based polymers and summarizedthe existing structureeproperty relationships of a wide variety ofPOSS-polymer systems. In particular, the state of aggregation orcrystallization of POSS moieties plays a prominent role in deter-mining physical properties of the POSS-based polymers. Theincorporation of POSS derivatives into polymeric materials leads toincreases in heat resistance, oxidation resistance, and surface andbulk hardness, improvement of mechanical properties, and reduc-tion in flammability and heat evolution.

Initial attention given to POSS-based copolymers, focused onrandom incorporation of POSS into amorphous host polymers,showed that POSS dispersed molecularly in the polymer matrix atlow POSS contents due to the good compatibility with the hosts buttended to aggregate and crystallize at high concentrations [27,28].On the other hand, studies by Coughlin and coworkers [29,30] onpolyethylene (PE)ePOSS copolymers having POSS moieties teth-ered randomly along linear chains as pendent groups showed thatPE and POSS crystallize separately to form phase-segregated crystaldomains at the nano-scale. In the crystallization process, one of thetwo crystallizable components (depending on the crystallizationcondition) crystallizes first and the other component then crystal-lizes in the spatially constrained, nano-confined regions. Suchcompetitive crystallization modulated the solid-state structure anddramatically impacted the physical properties. Beyond randomarchitecture, other researchers have reported on POSS-containinglinear polymers with different architectures such as telechelic co-polymers [31e34] and multiblock polymers, including diblock [35],and triblock [36] vinyl copolymers and multiblock TPUs [23,37,38].Blocky segment(s) consisting of multiple POSS moieties within asingle segment of diblock and triblock copolymers, adjacent toflexible, amorphous polymer segments, have been reported tocause nano-scale phase-separation and form self-assemblednanostructures [35,36], similar to conventional block copolymers.It was also shown that POSS did not crystallize in those materials, afinding attributed to the rigidity and conformational constraint inthe POSS-blocks [35,36]. Broadly speaking, nanostructure forma-tion occurs through competitive crystallization and is dependenton molecular interactions between POSS and polymer segments,POSS-block length, and the amount of POSS [24,29]. The mecha-nism for this competitive crystallization is not well understood.

Linear polymers of POSS-containing multiblock thermoplasticpolyurethanes (TPUs) can form useful nanometer scale structuresthat enhance thermal stability and mechanical properties of ma-terials. One-step polymerization is a common method employed toprepare POSS-containing multiblock TPUs through the use of apolymeric diol as soft segment, dihydroxy-functionalized POSS(POSS-diol) as a chain extender, and a stoichiometric amount ofdiisocyanate for polymerization. While convenient, this polymeri-zation approach occurs in a random fashion, yielding variation in

soft-block and hard-block (POSS-containing) lengths. Two step TPUpolymerization involving first end-capping the polyol with dis-ocyanate (forming a “pre-polymer”) followed by chain extensionwould allow additional control over the soft-block and hard-block(POSS) lengths along multiblock TPU chains, a method adopted inthis study.

For this study, we hypothesized that the kinetics of crystalliza-tionwill differ substantially by utilizing poly(ε-caprolactone) (PCL)-diols of varying molecular weight in a confined environmentsimilar to that created by POSS molecules. Crystallizable 2.6k and3.6k PCL- and POSS-containing multiblock TPU samples were pre-pared by a two-step polymerization using PCL-diol as a polyol andPOSS-diol as a chain extender to form alternating [AeB]n multi-block sequence structure (A: a single PCL, B: multiple POSS moi-eties). The aim of this studywas to quantitatively evaluate the effectof the multiblock sequence structure on crystallization behavior ofboth PCL and POSS segments as well as the resulting nano-structures. In particular, differential scanning calorimetry (DSC)and simultaneous wide-angle and small-angle X-ray scattering(WAXS/SAXS) measurements are used to investigate thecomposition-dependent phase behavior and crystalline nano-structures, respectively. Furthermore, we also report on an inter-esting and unexpected phenomenondnanostructure evolutiondinvolving both PCL and POSS blocks upon repeated thermal cycling.

2. Experimental section

2.1. Materials

Poly(ε-caprolactone) diol (PCL-diol) with nominal numbereaverage molecular weights (Mn) of w2000 and w3000 g/mol werepurchased from Scientific Polymer Products (USA). 1H NMR wasused to determineMn values (Mn, NMR) of these PCL-diols, revealingMn, NMR ¼ 2600 and 3600, respectively. The PCL-diols, referred to asPCL2.6k-diol and PCL3.6k-diol, respectively, were kept dry undervacuum at room temperature prior to use. Dibutyltin dilaurate(DBTDL) and hexamethylene diisocyanate (HDI) were purchasedfrom Aldrich and used without further purification and kept undera dry nitrogen atmosphere. 1-(3-(2,3-Dihydroxypropyl)oxy)propyl-3,5,7,9,11,13,15-isobutylpentacyclo[9.5.1.13,9.15,15.17,13]octasiloxane(or “1,2-PropanediolIsobutyl-POSS�”), hereafter referred to asPOSS-diol, was purchased from Hybrid Plastics as a pure (>99%)crystalline solid and kept under vacuum at room temperature priorto use. N-methyldiethanolamine (NMDEA) was purchased fromAldrich and dried using a hand-made column filled with molecularsieves (pore size: w4 �A) prior to use. Toluene was purchased fromFisher Scientific and distilled with calcium hydride prior to use.Tetrahydrofuran (THF), acetone, and hexane were purchased fromFisher Scientific and used as received. Structural compositions ofthe final polyurethane products were analyzed using liquid phase1H NMR spectroscopy for deuterated chloroform solutions with aVarian Inova spectrometer operating at 600 MHz and at roomtemperature.

2.2. Synthesis of PCLePOSS multiblock thermoplastic polyurethanes

The chemical synthesis of PCL- and POSS-containing TPUs wascarried out using PCL-diol, POSS-diol, NMDEA, HDI, and DBTDL us-ing a two-step polymerization procedure, where molar ratios ofPCL-diol, POSS-diol, and NMDEA in the feed were set to 1.0/2.0/2.0,1.0/3.0/2.0, and 1.0/4.0/2.0. Herewe describe the detailed procedureto prepare a TPU with a molar ratio of PCL3.6k-diol/POSS-diol/NMDEA ¼ 1.0/2.0/2.0 as a representative example. First, 5.00 g ofPCL3.6k-diol (1.39 mmol) were dissolved in freshly distilled toluene(20 mL) in a 100 mL Airfree round-bottom flask (ChemGlass) and

E. Huitron-Rattinger et al. / Polymer 54 (2013) 3350e33623352

kept under agitation and nitrogen purge at 70 �C for 30 min. Theend-capping of the PCL-diol was performed adding a toluene solu-tion of HDI, with 1.17 g (6.94 mmol) of HDI in 15 mL of toluene,leaving the reactants at 70 �C for 20 min under stirring. Next, 2.64 g(2.78 mmol) of POSS-diol and 0.33 g of NMDEA (2.78 mmol) weredissolved in 15 mL and 5 mL, respectively, of toluene and kept at70 �C (NMDEAwas selected as the chain extender in anticipation offuture studies concerningquaternizationof this group thatwill yieldionomeric polyurethanes.). The toluene solutions were then addeddrop-wise, first POSS-diol and secondly NMDEA. After these addi-tions, the reactor temperature was increased to 95 �C. The mixturewas left for 10 h under nitrogen purge and stirring. A highly viscousand transparent liquidwas obtained. The productwas then two-folddiluted with THF and precipitated in cold hexanes using a dry ice/acetonebath. Theprecipitatewas thendried inhighvacuumat roomtemperature for 72 h. An alternative method for precipitating theproducts, especially when using higher POSS contents, involvedcasting the solution onto a Teflondish, followedbydissolution of theresulting cast film in acetone at w50 �C with a saturated concen-tration, cooling down the acetone solution to�20 �C, and keeping itat the temperature overnight, so that the polymer products re-precipitated in the cold-acetone. A series of PCLePOSS TPUs ofvarying molecular weights were synthesized. The samples arenamed as follows: TPUnk_1-2, TPUnk_1-3, and TPUnk_1-4, where nis either 2.6 or 3.6, referring to PCL2.6k-diol and PCL3.6k-diol,respectively. The other digits indicate the feed molar ratios of PCL-diol and POSS-diol, respectively. 1H NMR of TPU3.6k_1-2 (CDCl3,600MHz): d (ppm)¼ 0.18 (d, (CH3)2CHeCH2eSi), 0.93 (s,eOeCH2e

CH2eN(CH3)eCH2eCH2eOe), 0.95 (s, (CH3)2CHeCH2eSi), 1.38 (m,eOeCH2eCH2eCH2eCH2eCH2eCOOe), 1.65 (m, eOeCH2eCH2e

CH2eCH2eCH2eCOOe), 2.35 (t, eOeCH2eCH2eCH2eCH2eCH2e

COOe), 2.70 (t, eOeCH2eCH2eN(CH3)eCH2eCH2eOe), 3.15 (q, eOOC(He)NeCH2eCH2eCH2eCH2eCH2eCH2eN(eH)COOe), 3.68 (t,eOeCH2eCH2eOeCH2eCH2eOe), 4.05 (t, eOeCH2eCH2eCH2e

CH2eCH2eCOOe), 4.15 (t, eOeCH2eCH2eN(CH3) eCH2eCH2eOe),4.22 (t, eOeCH2eCH2eOeCH2eCH2eOe). The other five TPU sam-ples (TPU2.6k_1-2, TPU2.6k_1-3, TPU2.6k_1-4, TPU3.6k_1-3, andTPU3.6k_1-4) exhibited nearly identical ppmvalues for these peaks.

2.3. Characterization

2.3.1. GPCGel permeation chromatography (GPC) was conducted using a

Waters Isocratic HPLC system equipped with a temperaturecontrolled differential refractometer (Waters 2414, held at 40 �C).Multi-angle laser light scattering was also employed (Wyatt mini-DAWN) using three angles (45�, 90�, 135�) for absolute molecularweight determination. THF solutions (2 mg/mL) were passedthrough a 0.2 mm PTFE filter prior to injection. The GPC was oper-ated at a flow rate of 1 mL/min and featured a series of three (3)columns of cross-linked polystyrene beads. The columns were 5 cm(first) and 30 cm long (second and third) ResiPore columns (Poly-mer Laboratories, Inc.), consecutively, each packed with 3 mm par-ticles designed for separations of polymers with molecular weightless than 400 kg/mol.

2.3.2. Thermal analysisPolymer phase transitions were characterized by differential

scanning calorimetry (DSC) under nitrogen atmosphere using TAInstruments Q100. Temperature and enthalpy calibrations werecarried out using analytical grade indium (Tm ¼ 156.6 �C) and zinc(Tm ¼ 419.5 �C). Samples weighing about 5 mg were loaded intostandard aluminum pans. For thermal cycles up to 140 �C, sampleswere heated to 140 �C at 10 �C/min and held there for 1 min to erasethe thermal history. Then, theywere cooled to�80 �C at�10 �C/min

(“1st cooling”), hold there for 3min, and re-heated to 140 �C at 10 �C/min (“1st heating”). This thermal cycle was repeated three times upto “3rdheating”. For runs up to 180 �C, sampleswereheated to180 �Cat 10 �C/min and hold there for 1 min to erase the thermal history.Then, they were cooled to �20 �C at �5 �C/min (“1st cooling”), heldthere for 5min, and re-heated to 180 �C at 10 �C/min (“1st heating”).This cycle was repeated seven times up to “7th heating”. Thermog-ravimetric analysis (TGA) was performedwith TA Instruments Q500at the heating rate of 10 �C/min under nitrogen atmosphere. Theonset of decomposition temperature, Tdec, was defined as the tem-perature where the samples showed 5% weight loss, and recorded.

2.3.3. WAXS/SAXS analysisTwo-dimensional simultaneous wide-angle X-ray scattering

(WAXS) and small-angle X-ray scattering (SAXS) patterns wereobtained using a three-pinhole collimation system, S-Max3000(Rigaku). This equipment employs a microfocus CuKa(l ¼ 1.5405 �A) radiation source, operated at 45 kV and 0.88 mA.WAXS patterns were recorded on Fujifilm image plates (IP) andFujifilm image reader FLA-7000 system, using a flat-plate cameraand a sample-to-detector distance of 6.0 cm. The patterns wereanalyzed using the software POLAR v2.6 (Stonybrook TechnologyInc., Stonybrook, NY). SAXS patterns were recorded using an areadetector and sample-to-detector distance of 1.55 m. Data wererecorded in the range 0.0054 < q < 0.16 Åe1, where q ¼ (4p/l)sinq,and 2q is the scattering angle. Compression-molded films of theTPU samples, prepared with two Teflon sheets and a Teflon spacer(thickness¼ 0.25mm) using a hot press at 5MPa at 140 �C for 5minor 180 �C for 2 min, were used for the WAXS/SAXS measurements.

2.3.4. FTIR spectroscopyMolecular bond identification was performed using an FTIR

Nicolet iS10 with ATR accessory, manufactured by Thermo Scien-tific. Spectra were recorded in attenuated total reflectance mode.The sample films were placed onto the ATR crystal and no addi-tional sample preparation was necessary to collect the spectra. Allspectra were acquired between 4000 and 400 cme1, at room tem-perature, carrying out 16 scans per spectrum, and at a spectralresolution of 2 cme1.

3. Results and discussion

3.1. Synthesis of PCLePOSS multiblock TPU samples

In this study, a series of six TPUs containingPCL segments of 2600or 3600 g/mol and POSS unitswith the PCL/POSSmolar ratios of 1/2,1/3, and 1/4 (TPUnk_1-2, TPUnk_1-3, and TPUnk_1-4; n¼ 2.6 or 3.6)were synthesized through the two-step polymerization including i)the chain end-capping of PCL-diol with HDI to formOCNePCLeNCOand ii) polymerization (chain extension) by the reaction amongOCNePCLeNCO, POSS-diol, NMDEA (the tertiary amine-diol chainextender), and HDI (Scheme 1). The actual molar ratios of PCL/POSS/NMDEA units, determined by 1H NMR, calculated weight percent-ages of the same, and the number-average molecular weight andpolydispersity, determined byGPC, of the resulting TPU samples aresummarized inTable 1.We expected that thefirst reaction step, PCL-diol with a large excess amount of HDI at 70 �C for 20 min in thepresence of tin catalyst would efficiently convert the chain-endfunctionality from hydroxyl (OH) groups to isocyanate (NCO)groups [39]. Thus, in the second step (polymerization), the bulkyPOSSmoieties and the tertiaryamineunits are insertedbetweenPCLsegments of the resulting TPUchains, inwhich themoieties adjacentto PCL segments must always be POSS units or the tertiary amineunits (there should be negligible PCLePCL sequence). Because theweight fraction of the non-crystallizable tertiary amine unit is very

OSi

O

SiO

Si

SiOSi

O

SiO Si

O

SiO

O

O

O

O

OHN N

HO

OHN N

HO

O

O

O

OO N

H

HN

O

ON

x y zOOOO

OO

m m

O

OOOOOO

m mO

HN NCO

O

HNOCN

O

PCL-diol (Mn = 2,600 or 3,600 g/mol)

DBTDL, toluene, 70 ºC, 20 min

OCNNCO

HDI

DBTDL, toluene, 95 ºC, 10 h

NMDEA

HON

OH

POSS-diol

TPUnk_1-2, TPUnk_1-3, or TPUnk_1-4 (n = 2.6 or 3.6)

OSi

O

SiO

Si

SiOSi

O

SiO Si

O

SiO

O

O

O

O

O OHOH

n

2n, 3n, or 4n

2n

5n, 6n, or 7n

PCL-diisocyanate

+ OCNNCO

HDI

n 3n, 4n, or 5n

OHOOHOOO

m mO

Scheme 1. Two-step polymerization of TPUnk_1-2, TPUnk_1-3, and TPUnk_1-4 (n ¼ 2.6 or 3.6).

E. Huitron-Rattinger et al. / Polymer 54 (2013) 3350e3362 3353

small (�3.4 wt.%), we can regard the TPU as amultiblock copolymerwith an alternating sequence structure of single-PCL and multiple(two, three, or four on average)-POSS blocky segments as illustratedin Fig. 1. We note that the actual molar ratios are very similar withthe feed ratios as listed in Table 1. Further, it is reiterated thatNMDEA is utilized with the intention of later studying

Table 1Characterization of PCLePOSS multiblock TPUs.

Sample Molar ratioa Weight contentb (wt.%)

PCL POSS NMDEA PCL POSS

TPU2.6k_1-2d 1.0 (1.0) 2.0 (2.1) 2.0 (1.6) 46.1 [42.7] 35.2 [4TPU2.6k_1-3d 1.0 (1.0) 3.0 (3.0) 2.0 (1.6) 38.9 [35.7] 43.4 [4TPU2.6k_1-4d 1.0 (1.0) 4.0 (4.1) 2.0 (1.4) 32.9 [29.5] 49.9 [5TPU3.6k_1-2e 1.0 (1.0) 2.0 (1.8) 2.0 (1.4) 58.2 [57.3] 27.1 [2TPU3.6k_1-3d 1.0 (1.0) 3.0 (3.4) 2.0 (1.5) 45.2 [41.6] 39.7 [4TPU3.6k_1-4d 1.0 (1.0) 4.0 (4.3) 2.0 (1.3) 40.4 [37.9] 45.0 [4

a Molar ratios in the feed and those of the obtained TPU chains. The values in the parb Weight contents in the obtained TPU chains (unbracketed) and those in the whole

analyses. The sums of weight contents are not 100 wt.% because the weight content of Hc Determined by GPC analysis.d Obtained by lowering temperature of concentrated acetone solution from ca. 50 �C te Obtained by dropping THF solution into excess cold hexane in a dry ice/acetone bat

quaternization of this group, which will render the polymerscationic with attending ionomeric properties.

In the GPC analysis, we observed that the synthesized TPUsamples contained some unreacted POSS-diol residue. The weightpercentages of POSS-diol residue in the materials were calculatedbased on the GPC analysis and are listed in Table 1. The weight

Mnc (g/mol) PDIc Weight content of POSS-diol

residuec (wt.%)NMDEA

0.0] 3.4 [3.2] 101,500 1.1 7.58.0] 2.8 [2.6] 40,100 1.1 8.25.1] 2.2 [1.9] 73,900 1.1 10.38.3] 2.7 [2.6] 87,600 1.1 1.74.5] 2.1 [2.0] 44,700 1.2 8.08.3] 1.7 [1.6] 28,500 1.2 6.1

entheses are the “actual” molar ratios of TPU chains determined by 1H NMR.materials including POSS-diol residue (bracketed) determined by 1H NMR and GPCDI-unit is excluded.

o �20 �C and keeping the temperature at �20 �C overnight.h.

TPUnk_1-2 (n = 2.6 or 3.6)

TPUnk_1-3 (n = 2.6 or 3.6)

TPUnk_1-4 (n = 2.6 or 3.6)

PCL2.6k or 3.6k

POSS

Fig. 1. Schematic of sequence structures of TPUnk_1-2, TPUnk_1-3, and TPUnk_1-4(n ¼ 2.6 or 3.6). The NMDEA unit is omitted in this cartoon for clarity.

emai

ning

Wei

ght (

%)

40

60

80

100

98

100

102

Temperature (oC)0 100 200 300 400 500 600

Rem

aini

ng W

eigh

t (%

)

0

20

40

60

80

100

100 150 200 250 300

96

98

100

102

(a)

(b)

i

ii

iii

i

ii

iiiiiii

Region I Region II

E. Huitron-Rattinger et al. / Polymer 54 (2013) 3350e33623354

percentages of POSS-diol residue were 1.7 wt.% for TPU3.6k_1-2and ca. 6e10 wt.% for the other samples, indicating that thedifferent precipitation methods strongly affected the efficiency ofPOSS-diol residue removal. Despite the existence of some unreac-ted POSS-diol in the precipitates, we characterized those sampleswithout further purification and consider the residue existence inour analysis and discussion.

Temperature (oC)0 100 200 300 400 500 600

R

0

20

100 150 200 250 300

96 ii

Fig. 2. TGA traces of (a) TPU2.6k series and (b) TPU3.6k series. Scans correspond to thePCL/POSS composition; (i) 1e2, (ii) 1e3, and (iii) 1e4. Enlarged views in the temper-ature range of 100e300 �C are also shown. TGA scans were carried out under nitrogenatmosphere at 10 �C/min.

3.2. Thermal decomposition behavior

Thermal decomposition behavior of the obtained TPU sampleswas examined with TGA at a heating rate of 10 �C/min in a nitrogenatmosphere. The Tdec of the reactants, PCL2.6k-diol, PCL3.6k-diol,and POSS-diol were 297 �C, 307 �C, and 231 �C, respectively (datanot shown). The relatively low Tdec for POSS-diol is due to subli-mation under the nitrogen atmosphere [40]. The weight loss ofPOSS-diol due to this sublimation starts at about 180 �C, with amore pronounced weight loss at 231 �C and ending point of about300 �C. Fig. 2 and Table 2 show the TGA traces and the Tdec valuesfor all TPU samples. We observed that the TPU samples eachexhibited a two-step weight loss sequence in the temperatureranges of ca. 200e390 �C (Region I, Fig. 2) and ca. 390e500 �C(Region II, Fig. 2) in the TGA curves, which are attributed to thesublimation of POSS moieties at the lower temperature region, andto PCL chain degradation at the higher temperature region. Thusthe Tdec values of the TPU samples listed in Table 2 correspond tothe onset temperatures of the sublimation of POSS moietiesreleased from both the TPU chains and the unreacted POSS-diolresidue. It is noted that TPU3.6k_1-2 with a much smalleramount of POSS-diol residue (¼1.7 wt.%) showed an obviouslyhigher Tdec value (308 �C) than the other samples (Tdec ¼ 278e290 �C) with larger amounts (¼6.1e10.3 wt.%). This data and Fig. 2indicate that the POSS-diol residue start the sublimation at about200 �C, while the sublimation of the POSS moieties of the TPUchains occurs at higher temperature region above ca. 250 �C afterurethane bonds start significant dissociation.

3.3. POSS-governed crystallization behavior below 140 �C

The effects of alternating sequence structure of two differentcrystallizable blocky components (PCL and POSS), and the PCL/POSS

segment lengths in the PCLePOSS multiblock TPU chains shown inFig. 1 are of significant interest in terms of crystallization of bothcomponents in nano-confined environments. DSC measurementswere performed in the temperature range spanning �80 �C to140 �C to evaluate the crystallization and melting behavior of bothblocks of the PCLePOSS multiblock TPU samples. In the DSC mea-surements, consecutive heating/cooling cycles at a heating and acooling rate of �10 �C/minwere repeated three times to ensure thethermal stability of the TPU samples. Indeed, it was observed thatthe heating/cooling traces of each of the TPU2.6k and TPU3.6k se-ries were consistent throughout the cycles, indicating those sam-ples are thermally stable in the temperature region up to at least140 �C.

As representative data, the 3rd cooling and the 3rd heatingtraces are shown in Fig. 3a and b, respectively, for TPU2.6k series,and in Fig. 4a and b, respectively, for TPU3.6k series. The temper-ature (Tmc) and enthalpy (DHmc) of melt-crystallization during the3rd cooling scan, those of cold-crystallization (Tcc and DHcc), thoseof melting (Tm and DHm), and glass transition temperature (Tg)during the 3rd heating scan are summarized in Table 2. As shown inFig. 3a and b, TPU2.6k series exhibited a sharp exothermic peakaround 120 �C and a relatively broad exothermic peak at a slightlylower temperature region of 93e105 �C during the cooling scan,

Table 2Thermal properties of PCLePOSS multiblock TPUs.

Sample Tmc (�C) DHmc (J/g) Tcc (�C) DHcc (J/g) Tm (�C) DHm (J/g) Tg (�C)a Tdec (�C)b

TPU2.6k_1-2 dc 0 dc 0 dc 0 �59, �20 28593, 118d 4.5 dc 0 128 4.7

TPU2.6k_1-3 dc 0 dc 0 dc 0 �58, �20 287101, 123d 6.4 dc 0 130 5.5

TPU2.6k_1-4 dc 0 dc 0 dc 0 �58, �20 290105, 120d 6.7 dc 0 129 6.7

TPU3.6k_1-2 �19 4.7 �34 10.0 35 21.5 �63 308116 3.5 dc 0 131 3.7

TPU3.6k_1-3 �7 1.5 �31 3.0 37 9.8 �63 28198, 121d 4.1 dc 0 130 4.0

TPU3.6k_1-4 dc 0 �31 0.2 34 2.1 �62 278101, 119d 5.3 dc 0 129 5.3

a Glass transition temperature during the DSC 3rd heating scan at 10 �C/min (top: PCL phase, bottom: POSS phase).b Thermal decomposition onset temperature determined by TGA.c Not observed.d Two exothermic peaks were observed.

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and only a single endothermic peak at about 130 �C, regardless ofthe POSS content during the heating scan. The two exothermicpeaks during the cooling and the single endothermic peak duringthe heating are associated with crystallization and melting,respectively, of POSS moieties (confirmed by WAXS, see below).This melting temperature (ca. 130 �C) is approximately 30 �C belowthe melting temperature of the reactant, POSS-diol (data notshown). We attribute this observation to the notion that POSSmoieties covalently connected to TPU chains should form relativelysmall crystallites compared with the molecular POSS. Peak toppositions and relative peak areas of the two exothermic crystalli-zation peaks (Tmc, POSS-1 and Tmc, POSS-2) on cooling are differentamong the three TPU2.6k samples as shown in Fig. 3a. Theappearance of two different crystallization peaks, i.e., heteroge-neous crystallization of POSS phase should be attributed to thepresence of free POSS-diol residue. It is reasonable to consider thatthe free POSS molecules act like a nucleating agent to initiate thecrystallization of POSS moieties of TPU chains, resulting in thesharp Tmc, POSS-1 peak at the higher temperature. The emergence ofrelatively broad, second crystallization peak at the lower temper-ature (Tmc, POSS-2) suggests that the distribution of POSS-diol resi-dues in TPU2.6k samples is not uniform, that is, the nano-confinedPOSS phase does not always contain the free POSS-diol molecules.Indeed, the peak area of broad crystallization peak (Tmc, POSS-2)relative to the sharp peak (Tmc, POSS-1) gradually increased with anincrease in POSS content in TPU2.6k series with similar POSS-diolresidue contents (7.5e10.3 wt.%). This is presumably because thenumber of POSS-diol residue-free POSS nano-domains increaseswith increasing the POSS content in TPU2.6k series.

The PCL segments in TPU2.6k series remained completelyamorphous and showed neither melting nor crystallization peaksduring thermal cycling, which is attributed to the significant chainmobility restriction in the nano-confined regions between the POSSsegments. It is also noted that TPU2.6k series showed two Tg valuesat ca.e58 �C and at�20 �C, almost irrespective of PCL/POSS contentsas shown in Fig. 3b. This indicates that these samples have twodifferent, phase-separated domains with different chainmobility inthe amorphous region.Oneof the twophases exhibiting the lower Tg(Tg,soft) close to thatof neat PCLpolyol, shouldbe ascribed to a PCL (orat least PCL-rich) phase,while the other phase showing thehigher Tg(Tg,hard) is ascribable to more rigid phase with restricted chainmobility, i.e., hard-blockof TPUchains, the chainmobilityofwhich isrestricted by hydrogen-bonding and POSS crystallites.

As for TPU3.6k series, as shown in Fig. 4a and b, the POSS phaseexhibited a similar tendency with that of TPU2.6k series; that is, asharp crystallization peak and a relatively broad crystallization

peak appeared at ca. 120 �C and at a slightly lower temperature,respectively, during the cooling scans (except for TPU3.6k_1-2), anda single melting peak appeared at ca. 130 �C, regardless of the PCL/POSS contents. TPU3.6k_1-2 showed only a single POSS crystalli-zation peak at 116 �C during the cooling, which is due to the lowcontent (1.7 wt.%) of free POSS-diol residue.

Because PCL crystallizes by chain folding, lower molecularweight PCL segments have difficulty in crystallizing [41], especiallywhen end-linked in a multiblock format. In contrast, as shown inFig. 4a and b and Table 2, the TPU3.6k series incorporating thelonger PCL polyol exhibited a Tg value of PCL phase at ca. e62 �C,distinctly lower than those of TPU2.6k series (indicating higherchain mobility of the former), and showed crystallization of PCLphase. The PCL phase showed a small and broadmelt-crystallizationpeak during the cooling scan, and the DHmc values became signifi-cantly smaller with increasing POSS content. Eventually the PCLphase lost the melt-crystallization peak (for TPU3.6k_1-4). Instead,the TPU3.6k series showed a cold-crystallization peak at a tem-perature approximately 30 �C higher than Tg during the heatingscans as a main crystallization peak (Fig. 4b).

The glass transition of hard-block, observed for TPU2.6k series,was not observed for TPU3.6k series due to overlapping with thecold-crystallization peak of PCL phase. As listed in Table 2, the sumof enthalpies of two crystallization peaks (DHmc þ DHcc) of PCLphase was not consistent with that of melting peak (DHm) of PCLphase, which is presumably because the DHcc value is not precisedue to the overlapping with glass transition of hard-block.

The melting endotherm for the PCL phase appeared at ca. 35 �C,and decreased significantly with larger POSS loading: decreasingfrom 21.5 J/g (for TPU3.6k_1-2) to only 2.1 J/g (for TPU3.6k_1-4).Despite the fact that TPU3.6k_1-4 has a significant PCL content (ca.40wt.%)we surmise, as later confirmedbyWAXS/SAXSanalysis, thatPOSS crystallization governs the crystallization of all the TPU sam-ples in the 3.6k series, overwhelming that of PCL. Collectively, thesedata indicate the TPU3.6k series features PCL segments that, whilethey can crystallize owing to the relatively high chain mobilitycompared with those of TPU2.6K series, still experience restrictionfrom crystallizing under the nano-confined environment.

We sought to further understand the DSC results through ex-amination of the crystalline structure in the TPUs, which wasprobed using WAXS. The WAXS patterns (azimuthally averagedfrom isotropic patterns) for the TPU2.6k series, thermally cycledand recrystallized, are shown in Fig. 5. The 2DWAXS pattern shownas an inset corresponds to TPU2.6k_1-2, and it is a typical Debye-Scherrer pattern of an unoriented semicrystalline polymer. Thepattern exhibits inner crystalline reflections and an outer

Temperature (oC)-100 -50 0 50 100 150

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Fig. 4. DSC traces of TPU3.6k series during (a) the 3rd heating, and (b) the 3rd coolingscans. Heating and cooling scans were carried out in the temperature range of �80 �Cto 140 �C at a heating and a cooling rate of �10 �C/min under nitrogen atmosphere; (i)TPU3.6k_1-2, (ii) TPU3.6k_1-3, and (iii) TPU3.6k_1-4.

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Fig. 3. DSC traces of TPU2.6k series during (a) the 3rd cooling, and (b) the 3rd heatingscans. Cooling and heating scans were carried out in the temperature range of �80 �Cto 140 �C at a heating and a cooling rate of �10 �C/min under nitrogen atmosphere; (i)TPU2.6k_1-2, (ii) TPU2.6k_1-3, and (iii) TPU2.6k_1-4.

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amorphous halo. The intensity around the azimuth is uniform,indicating that the samples are unoriented and this was true for allthe TPUs. The background-subtracted, azimuthally averaged radialscans show that the smaller angle crystalline reflections correspondto POSS rhombohedral structure. Consistent with DSC results forthe TPU2.6k series, there is no evidence of PCL crystalline re-flections, which would be located at 2q ¼ 21.4�, corresponding to110 reflection of an orthorhombic structure. Instead, there is only abroad amorphous halo at 2q ¼ w18�. These results agree with thethermal analysis results shown in Fig. 3a, where the single endo-therm was associated with the melting of POSS crystals.

The nanostructure of the TPU2.6k series was further investi-gated via SAXS, and the 2D patterns and intensity traces for theTPU2.6k series are shown in Fig. 6. The patterns exhibit discrete, yetweak and broad scattering maxima. This indicates the existence oflong-range order. This can be easily appreciated from the corre-sponding azimuthally averaged radial intensity traces, also shownin Fig. 6. Note that the intensity maximum gradually moves towardthe beamstop as the concentration of POSS increased in the TPUs,indicating that at higher POSS concentration the long spacing islarger. The nanoscale order was readily solved from the Lorentz-

corrected intensity data (see Supplementary Material Figure S1).For the TPU2.6k_1-2, two scattering maxima were observed atq ¼ 0.059 Åe1 and 0.125 Åe1, corresponding to the long periods (d)of 108 �A and 50 �A. These are considered to be the first-order andsecond-order scattering maxima, whose scaling corresponds to alamellar nanophase [42]. Based on the DSC and WAXS results, wereason that the lamellar nanophase corresponds to POSS crystallineaggregates. This morphology, typical of symmetric AeB diblockcopolymers, is not unexpected as the blocky PCL and POSS seg-ments in the multiblock TPU2.6k_1e2 have similar molecularweights (approximately 2,600 g/mol).

For the compositionally more asymmetric polyurethaneTPU2.6k_1-3 (POSS block larger than PCL block see Fig. 1), twoscatteringmaximawere alsoobservedat q¼0.052Åe1 and0.107Åe1,corresponding to d ¼ 120 �A and 60 �A. These are again a first andsecond order scattering from a POSS lamellar nanostructure.

Finally, for TPU2.6k_1e4, the long-range order structure greweven further, the first-order scattering maximum shifting toq ¼ 0.046 Åe1 which corresponds to d ¼ 136 �A. Furthermore, theLorentz-corrected intensity traces exhibits more scattering max-ima, at q ¼ 0.082 Åe1, 0.12 Åe1, and 0.165 Åe1, corresponding tod ¼ 76 Å, 52 �A, and 38 �A. The positions of the secondary maximarelative to the first-order reflection (136�A) follow the scaling: 31/2,

Fig. 7. WAXS intensity traces of (a) TPU3.6k_1-2, (b) TPU3.6k_1-3, (c) TPU3.6k_1-4, (d)PCL3.6k-diol and (e) POSS-diol. Inset shows WAXS pattern of TPU3.6k_1-2. CuKaradiation.

Fig. 5. WAXS intensity traces of (a) TPU2.6k_1-2, (b) TPU2.6k_1-3, (c) TPU2.6k_1-4, (d)PCL2.6k-diol and (e) POSS-diol. Inset shows 2D WAXS pattern of TPU2.6k_1-2. CuKaradiation.

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71/2, and 131/2 which corresponds to a cubic nanophase [42].Indeed, the cubic nanophase composed of POSS crystalline aggre-gates has been previously reported for a highly symmetric PCLePOSS network polymer [41].

The nanostructure of the TPU3.6k series thermally cycled andrecrystallized was also investigated, anticipating a competingcrystallization environment based on DSC results for the sameTPU3.6k series. Fig. 7 shows the WAXS azimuthally averaged in-tensity traces of the TPU3.6k series. The 2DWAXS pattern shown inthe inset corresponds to the TPU3.6k_1-2 sample. For this series,the patterns exhibit concentric crystalline reflections correspond-ing to coexisting POSS and PCL crystals. The presence of PCL crystalsis consistent with the double endothermic peaks observed via DSC(see Fig. 4a, trace i). However, we observe that as the concentrationof POSS increases, the crystallinity of PCL becomes increasinglysuppressed (see Fig. 7, trace c). Again, these results are consistent

Fig. 6. SAXS patterns and azimuthally averaged radial intensity traces of (a)TPU2.6k_1-2, (b) TPU2.6k_1-3, and(c) TPU2.6k_1-4. CuKa radiation.

with the gradual reduction of PCL endothermic peak (see Fig. 4a,trace iii), and indicate that POSS crystallization overwhelms thecrystallization of PCL at higher concentrations of POSS. The crys-tallization behavior of TPU3.6k series is distinctly different from theTPU2.6K series, and this is clearly attributed to PCL molecularweight.

The nanostructure of the TPU3.6k series was further investi-gated via SAXS, and the results are shown in Fig. 8. The patternsalso exhibit discrete scattering maxima that indicate the existenceof long-range ordered structure compared to those of TPU2.6kseries (Fig. 7). The corresponding azimuthally averaged radialintensity traces are shown in Fig. 8. The nanoscale order wasagain readily solved from the Lorentz-corrected intensity data(see Supplementary Material Figure S2). For TPU3.6k_1-2, three

Fig. 8. SAXS patters and azimuthally averaged radial intensity traces of (a) TPU3.6k_1-2, (b) TPU3.6k_1-3, (c) TPU3.6k_1-4. CuKa radiation.

Fig. 9. DSC traces during the heating scans (10 �C/min) up to 180 �C as a function ofthermal cycle for (a) TPU2.6k_1-2, (b) TPU2.6k_1-3, and (c) TPU2.6k_1-4. Thermalcycles are numbered on each trace.

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scattering maxima were observed at q ¼ 0.0409 Åe1, 0.083�A�1, and0.120 Åe1, corresponding to the long periods (d) of 153 Å, 76�A, and52 �A. These are considered to be the first-order, second-order andthird-order scattering maxima, whose scaling corresponds to alamellar nanophase. The DSC results (Fig. 4a) showed that theendothermic peak of PCL is much more intense than POSS endo-thermic peak. Thus, we deduce that the lamellar nanophase inTPU3.6k_1-2 corresponds to PCL crystalline structure.

On the other hand, the SAXS results of TPU3.6k_1-3 (Fig. 8b.Figure S2) showed scattering maxima at q ¼ 0.035 Åe1, 0.119 Åe1,and 0.165 Åe1, corresponding to the long periods (d) of 177 Å, 53�A,and 38 �A. The positions of the secondary maxima relative to thefirst-order reflection (177 �A) follow the scaling: 121/2 and 201/2

corresponding to a cylindrical hexagonal nanophase [43].Finally, Fig. 8c shows the SAXS pattern of TPU3.6k_1-4. This

sample also shows long-range order structure with scatteringmaxima at q¼ 0.047 Åe1, 0.108 Åe1, and 0.166 Åe1 corresponding tod ¼ 134 Å, 58 �A, and 38 �A. The positions of the secondary maximarelative to the first-order reflection (134 �A) follow the scaling: 51/2

and 121/2, also consistent with a cylindrical hexagonal nanophase[43]. Note thatWAXS showed no evidence of a PCL crystalline phase(Fig. 7c). Although the DSC heating trace (Fig. 4a, trace iii) stillsshows a melting endotherm associated with PCL, the endotherm ismuch diminished relative to the other two TPU3.6k series with lessPOSS content. Thus, it appears that POSS dominates the nanophasefor this sample, hindering the ability of PCL to crystallize.

3.4. Phase behavior above 180 �C: reversible urethane bonding

Given the thermally reversible nature of the urethane bonding,the equilibrium of the reversible reaction (ReNCO þ HOeR0 4 ReNHCOOeR0), in which the bond formation is exothermic and thedissociation is endothermic, should be dominated by temperature.Thermodynamically, bond formation and the dissociation shouldbe predominant at lower temperatures and at higher temperatures,respectively. It is reasonable to consider that at an intermediatetemperature region the urethane bonds can experience dynamicdisconnect/reconnect behavior. For lower molecular weight ure-thane compounds [44,45], the dissociated small molecules arevolatile, which makes them difficult to reconnect. On the otherhand, polyurethanes may often experience higher temperatures(150e220 �C) [45] and reversibility of urethane bonds in suchpolyurethanes experience disconnection/reconnection of urethanebonds [46].

As noted above, aliphatic urethane bonds of our TPUs startsignificant dissociation at ca. 250 �C during the heating at 10 �C/min(TGA results, Fig. 2). Thus, the reversible disconnection/reconnec-tion of urethane linkages in these particular systems may occurbelow ca. 250 �C. Since free POSS-diol sublimes above ca. 200 �C asshown above, here we examined thermal treatment of the TPUsamples up to 180 �C to see if the dynamic reversible reaction ofurethane linkages occurs and, if it does, to what extent it affects themultiblock polyurethane structure. Consecutive heating/coolingthermal cycles up to 180 �C for TPU2.6k and TPU3.6k series wereconducted and repeated seven times using DSC. Thermal propertiesand crystalline structures were evaluated and comparedwith thosein the case of thermal cycles up to 140 �C shown above.

The TPU samples were heated up to 180 �C at 10 �C/min andcooled down to �20 �C at �5 �C/min in the thermal cycles. Fig. 9shows the DSC heating traces obtained for the TPU2.6k series.The appearance of a small shoulder in the melting peak in Fig. 10bcould be attributed to the presence of different crystallite sizes ofPOSS, a crystal-to-crystal transition or two coexisting crystallinestructures [47]. Excellent reproducibility of the thermal phasetransition behavior was observed throughout the seven cycles,

indicating that the microstructure did not change. This indicatesnegligible thermal degradation of urethane bonds and thus nosequence structure change.

For comparison, Fig. 10 shows the DSC heating traces obtainedby seven thermal cycles for the TPU3.6k series. The results showthat the heating/cooling cycles up to 180 �C for TPU3.6k seriesinduce a change of crystalline structures, along with the appear-ance of the third endotherm approximately 10 �C above the originalPOSS melting endothermic peak. In particular, with continuedthermal cycling, there is first a slight narrowing of the low and hightemperature endotherms and eventually an emergence of the third

Fig. 10. DSC traces during the heating scans (10 �C/min) up to 180 �C as a function ofthermal cycle for (a) TPU3.6k_1-2, (b) TPU3.6k_1-3, and (c) TPU3.6k_1-4. Thermalcycles are numbered on each trace.

E. Huitron-Rattinger et al. / Polymer 54 (2013) 3350e3362 3359

crystalline phase with slightly higher Tm. In order to address thequestion of thermal degradation, the molecular weight of eachsample was measured after thermal cycling by GPC. For instance,for the TPU3.6k_1-3 sample, the molecular weight for the as-polymerized sample was Mn ¼ 44,700 g/mol. After thermal treat-ment at 140 �C, 160 �C, and 180 �C for 10 min, GPC showedMn ¼ 57,500 g/mol, 44,900 g/mol, and 51,900 g/mol, respectively.These are not considered to be significant changes. Furthermore,FTIR studies were conducted to monitor whether thermal cycling

would cause thermal degradation (see Supplementary MaterialFigure S3). Typical phase-segregated polyurethane signals consis-tent with the previous report by Sung and Schneider [48] wereobserved. The FTIR results showed that there is no chemicaldecomposition and that the thermal cycles had no impact what-soever on the chemical stability of the samples. Together, theseresults indicate that the microstructure of the TPU3.6k serieschanged by the thermal cycles up to 180 �C, and that thermallyinduced chain rearrangement preserving chemical compositionand molecular weight was the cause.

It is reasonable to postulate that the formation of larger andmoreordered crystallites of both PCL and POSS are responsible for theappearance of the third melting endothermic peak (Fig. 10a), whichshould be associatedwith POSS crystallites, and the slight narrowingof melting peaks of both PCL and POSS phases. We attribute this tothe intra-/inter-chain rearrangements associated with dynamicdisconnection/reconnection of urethane bonds, resulting in longerPCL and POSS blocks (including increasing numbers of the samespecies within the blocks, such as .PCLePCLePCL. and .POSSePOSSePOSSePOSSePOSS.) along the TPU chains. The difference inthe influence of the thermal cycles between TPU2.6k and TPU3.6ksamples is attributed to different thermal stability: the TPU3.6k se-ries with longer PCL segments and lower local POSS densities alongthe chains is evidently more subject to dissociation of urethanebonds at the thermal conditions we adopted.

The timing (in cycle number) of the appearance of the thirdendotherm for TPU3.6k samples during the repeated thermal cyclesdepended strongly on the POSS content; it appeared in the 1st cyclefor TPU3.6k_1-2, and TPU3.6k_1-3, and in the 4th cycle forTPU3.6k_1-4. The increase in POSS contentwithin theTPU3.6k seriesapparently retards the onset of urethane bond dissociation. This isreflected in themuchdelayed appearance of the third endotherm forTPU3.6k_1-4 than for the other TPU3.6k samples. The much earlierappearance of larger and sharper third endotherm for TPU3.6k_1-3than for TPU3.6k_1-2, shown in Fig. 10a and b may be due to thelowermolecularweight (and thus viscosity) of the former sample. Incontrast to what is being observed for POSS phase, Fig. 10c showsinconsistent appearance of a melting peak associated with the PCLcrystalline phase. This behavior could be given by the increasingchain mobility restriction imposed by POSS crystallization whichhinders PCL ability to crystallize by chain folding, despite thedissociation and reconnection of the urethane linkages.

The DSC results for the TPU3.6k samples, especially forTPU3.6k_1-2 and TPU3.6k_1-3, also showed that the endothermassociated to PCL melting became more pronounced after eachthermal cycle. These results suggest the elimination of defects inPCL crystalline structure after each recrystallization step. This wasconfirmed by the X-ray scattering results shown in Fig. 11, anddiscussed below.

The DSC results obtained for the TPU3.6k series were correlatedwith the micro- and nanostructure as investigated via X-ray scat-tering, wide-angle and small, respectively. Fig. 11 shows azimuth-ally averagedWAXS intensity traces of thermally cycled TPU3.6k_1-2. WAXS data for (a) PCL3.6k-diol and (e) POSS-diol are alsoincluded for the sake of comparison. The intensity traces wereobtained after (b) 1st, (c) 5th and (d) 7th thermal cycles. Trace (a)shows the intensity trace for the neat PCL: there are crystallinereflections at 2q ¼ 21.4� and 23.66� (d ¼ 4.15 �A and 3.76 �A,respectively), corresponding to (110), and (200) lattice planes of anorthorhombic unit cell [41,49]. The intensity trace for POSS-diol(trace e) exhibits reflections at 2q ¼ 7.9�, 10.6�, and 18.6�

(d ¼ 11.2 Å, 8.3 �A, and 4.8 �A, respectively) which correspond to arhombohedral crystal unit cell [43]. The results show quite inter-estingly that thermal cycling induced crystallization of PCL (tracesc, d); i.e., the thermal treatment induced the coexistence of the

Fig. 11. WAXS intensity traces of (a) PCL3.6k-diol, TPU3.6k_1-2 after (b) 1st, (c) 5th,and (d) 7th thermal cycles, and (e) POSS-diol. Inset shows 2D WAXS pattern ofTPU3.6k_1-2 after 7th thermal cycles. CuKa radiation.

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crystalline phases of PCL and POSS within the TPU. While the 2.6kseries featured crystallinity owed entirely to POSS, the 3.6k seriesowes its crystallinity to both PCL and POSS.

The coexistence of crystalline phases has been previously re-ported for the highly symmetric POSS-PCL-diols studied byAlvarado-Tenorio et al. [41,50]. Furthermore, the present resultsshow that the original POSS-diol reflection at 2q ¼ 18.6� shifted tohigher scattering angle values of 2q ¼ 19.57�. The result indicatesthat POSS unit cell has been distorted from its unit cell parametersto a constrained morphology, given the prevailing PCL crystalliza-tion. Note that SAXS had shown that PCL lamellar morphologydominates the nanophase in this particular sample (see Fig. 8a anddiscussion above).

Increasing the concentration of POSS in the TPU3.6k series led toa different response to thermal cycling. Fig. 12 shows azimuthallyaveraged WAXS intensity traces of thermally cycled TPU3.6k_1-3.The WAXS intensity traces were obtained after: (b) 1st, (c) 5thand (d) 7th thermal cycles and data for (a) PCL3.6k-diol and (e)POSS-diol are again included for the sake of comparison. There aretwo interesting results in this case: (i) as for TPU3.6k_1-2, there isthermally induced crystallization of PCL (the 110 and 200 re-flections are clearly visible), and (ii) strikingly, these reflections are

Fig. 12. WAXS intensity traces of (a) PCL3.6-diol, TPU3.6k_1-3 after (b) 1st, (c) 5th, and(d) 7th thermal cycles, and (e) POSS-diol. Inset shows 2DWAXS pattern of TPU3.6k_1-3after 7th thermal cycle. CuKa radiation.

slightly shifted to smaller angles, denoting an increase in the crystallattice dimensions (scattering angle and d-spacing are reciprocal).The weakness of PCL crystalline reflections denotes the difficultyfor PCL to crystallize, and this is correlated with the correspond-ingly small melting endotherms shown in Fig. 12b. Moreover, theincrease in d-spacing suggests that PCL crystal lattice is stressedupon crystallization. Note that POSS crystalline reflections remainat constant 2q positions throughout the thermal cycles, denotingthe dominance of POSS crystalline phase in this TPU, as discussed inthe previous section (see Figs. 5b and 8b and corresponding dis-cussion of results).

Finally, Fig. 13 shows the WAXS intensity traces for theTPU3.6k_1-4 sample. The results show POSS crystalline reflections,and these are not shifted in their 2q positions. Moreover, after fivethermal cycles PCL has not crystallized; it is apparently unable tocrystallize to a size large enough to yield detectable diffraction. Theintensity traces for 1st, 3rd, and 5th thermal cycles are identical.Correspondingly, the DSC heating traces shown in Fig. 10c revealdifficulty for PCL to crystallize as its endotherm appears and dis-appears with increasing thermal cycles.

Competing crystallization between POSS and PCL is evidencedthroughout the thermal cycles carried out on the TPU3.6k series aswemodified the soft segment content and POSS concentration. TheWAXS results shown in Figs. 10e13 may be rationalized as follows.TPU3.6k_1-2 bearing the lowest concentration of POSS shows thatthe reflection at 2q ¼ 18.6� was shifted to higher scattering anglevalue of 2q ¼ 19.57�. Thus, the POSS unit cell was distorted to aconstrainedmorphology, given the prevailing PCL crystallization. Inaccordance, it is this system, TPU3.6k_1-2, which displayed themost intense PCL reflections (Fig. 11a and d).

For the TPU3.6k_1-3, the POSS crystalline reflections remainedunchanged, nonetheless PCL reflections were shifted towards lowerangles, which indicates that the unit cell was distorted from itsoriginal unit cell parameters presumably by the “pull” of POSScrystals and in order to satisfy the new conditions for its owncrystallization. This may be due to the fast crystallization of POSSthat restrains the mobility of PCL chains.

As POSS concentration was further increased in sampleTPU3.6k_1-3 (Fig. 12), the PCL crystalline reflections diminished(traces c and d) until completely fading away for TPU3.6k_1-4(Fig. 13). The trend is rather clear: as more POSS was added to theTPU, while the relative molar ratios of PCL and NMDEA were keptfixed, POSS crystallization was favored. Table 3 summarizes theWAXS results. For the TPU3.6k series, the thermal treatmentspromoted rearrangement of the structure at a certain extent, being

Fig. 13. WAXS intensity traces of (a) PCL3.6k-diol, TPU3.6k_1-4 after (b) 1st, (c) 3rd,and (d) 5th thermal cycles, and (e) POSS-diol. Inset shows 2D WAXS pattern ofTPU3.6k_1-4 after 7th thermal cycle. CuKa radiation.

Table 3WAXS reflections for thermally cycled TPU3.6k series.

Sample SSC% POSS% Expected POSSreflections (2q)

Observed POSSreflections (2q)

Expected PCLreflections (2q)

Observed PCL reflections (2q)

TPU3.6k_1-2 75.34 27.1 18.6 19.57 21.25 No shifting23.66

TPU3.6k_1-3 77.74 39.7 18.6 No shifting 21.25 20.5823.66 22.91

TPU3.6k_1-4 79.21 45.0 18.6 No shifting 21.25 PCL reflections vanished23.66

0 0.05 0.1 0.15 0.2 0 0.05 0.1 0.15 0.2 0 0.05 0.1 0.15 0.2

(a)

Inte

nsit

y (a

.u.)

(b)

Inte

nsit

y (a

.u.)

(c)

Inte

nsit

y (a

.u.)

Fig. 14. SAXS intensity traces of (a) TPU3.6k_1-2, (b) TPU3.6k_1-3, and (c) TPU3.6k_1-4 after (ii) 1st, (iii) 3rd, and (iv) 5th thermal cycles. PCL3.6k-diol is shown as trace (i) in eachgraph for comparison.

E. Huitron-Rattinger et al. / Polymer 54 (2013) 3350e3362 3361

these rearrangements totally dependent on heating/cooling con-ditions. By carefully adjusting heating/cooling rate parameters, itshould be possible to modulate the degree of crystallinity of thesematerials.

Changes to long-range order by urethane bond rearrangementwere examined with SAXS. SAXS patterns shown in Fig. 14demonstrate that POSS and PCL are not only able to crystallize,but also form aggregates at the nano-scale, with structural detailsdepending on POSS content. The results show that after eachthermal cycle the long-range periodicity has increased (scatteringmaxima are being shifted to smaller q values), the growth of thenanophase even extending beyond the range of the instrument. It isnoted that the growth of crystal nanophases occurs for both PCLcrystal bundles (Fig. 14a) and POSS crystal aggregates (Fig. 14b andc). As these results were carried out after thermally cycling thesamples (post-mortem analyses) further experimentation is underway in order to investigate, in-situ, the influence of the thermalcycling on the micro- and nano-structure.

4. Conclusions

A series of novel PCL- and POSS-basedmultiblock polyurethaneswith alternating sequence structures of PCL segments with 2600 or3600 g/mol and multiple-POSS blocky segments (TPU2.6k andTPU3.6k series, respectively) were synthesized via two-step poly-merization. Through simultaneous WAXS/SAXS measurements wewere able to study the nanostructures of the TPU samples as well asthe nanostructure evolution induced by thermal treatments. Wepresented a rationale explanation on how crystallinity developedover the course of several heating/cooling cycles, where thechemical composition and ratio of the constituent segments as wellas processing conditions, can lead to themanipulation of the degree

of crystallinity with ulterior tailoring of bulk properties. In themultiblock TPUs, POSS phase crystallized first during the coolingfrom themolten state. Crystallization of PCL phasewas significantlyrestricted in the nano-confined environment between POSS seg-ments. POSS moieties assembled into higher-ordered lamellar, cy-lindrical, or cubic nanophase crystalline structures, depending onPOSS incorporation level, as detected by SAXS. Repeated consecu-tive heating/cooling thermal cycles in the temperature range up to180 �C caused the formation of larger and more ordered crystallinestructures for both crystallizable components. This was attributedto the dynamic dissociation/reconnection behavior of urethanebonding in the PCLePOSS multiblock TPU samples at high tem-perature region up to 180 �C, which seems to cause the intra-/inter-chain rearrangement of highly mobile PCL and bulky POSS seg-ments and the resulting increase of PCL and POSS block lengths inthe TPU chains.

Acknowledgments

The authors gratefully acknowledge the help of Dr. B. Alvarado-Tenorio (Syracuse University) with X-ray scattering experiments aswell as enlightening discussions. E. Huitron-Rattinger was sup-ported by a graduate scholarship from the Mexican Council forScience and Technology (CONACyT). This research was partiallysupported by CONACyT (CIAM2008 program, grant 107294 andCiencia Basica 2011 program, grant 168095) and the NSF under theMaterials World Network (DMR-0758631) programs.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.polymer.2013.04.015.

E. Huitron-Rattinger et al. / Polymer 54 (2013) 3350e33623362

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