Crystallization of hard segments in MDI/BD-based polyurethanes deformed at elevated temperature and their dependence on the MDI/BD content

Highlights

• Strain-induced crystallization of amorphous hard segments of TPU with low HSC is proposed.

• Para-crystalline hard segments of TPU with high HSC transform into form III of MDI/BD modifications.

• Formed microfibrils exhibit lateral periodic correlation.

• Thickness of hard domain can be elongated at 100 °C.

Abstract

Morphological variation of thermoplastic polyurethanes (TPUs) with different hard segment contents (HSCs) stretched at 100 °C higher than the T_g of hard domain was monitored by in-situ wide-angle X-ray scattering (WAXS) and small-angle X-ray scattering (SAXS) techniques. WAXS patterns reveal that the crystalline modification of TPUs with high HSCs transforms from paracrystal (termed as form I) to form III of 4,4′-methylene diphenyl diisocyanate/1,4-butanediol (MDI/BD) segment, and TPUs with low HSCs show the direct transition from amorphous MDI/BD packing to form III packing. The form III crystals are fibrillar hard domains. Chord distribution functions (CDFs) computed from the SAXS patterns were employed to illustrate the domain topology. The streak-like CDF peaks off the meridian indicate the existence of cylindrical microfibrils which are composed of very low inhomogeneities between the destructured hard domains and stretched soft domains along the straining direction. These microfibrils of TPUs with low HSCs present a periodically lateral correlation. Interface distribution function (IDF) and chord length distribution (CLD) were computed from Bonart's longitudinal and transverse projection to quantify the domain stacking and microfibril arrangement respectively. The average size of hard domains increases up to a constant size with the increasing macroscopic strain which may be related to the maximal sequence length of hard segment. The average size of soft domain also reaches up to a constant size at large strain that is smaller than the fully-extended length of soft segment. The variation of domain size is different from deformation at room temperature. The HSC affects the behaviors of morphological evolution significantly.

Introduction

Thermoplastic polyurethane (TPU) elastomer is applied vastly in automobile, sporting goods, coatings, roller, textile, health-care products, and so on [1], [2], [3]. TPU is a kind of multi-block copolymer synthesized with diisocyanate, chain extender diol and macrodiol. Generally diisocyanate and diol compose hard segment. Macrodiol forms soft segment. Material properties can be tuned by substituting certain component and controlling the ratio of different components [4], [5] and even the chemical reaction condition [6], [7], [8], [9], [10], [11], [12], [13]. One-shot process is frequently applied in industry to reduce the cost. Different reactivity of OH-groups in diol and polyol determines their random reaction with NCO-group in diisocyanate. Random reaction of these three components induces random covalent-bonded MDI/BD modules in the chain. Therefore, length distribution of hard segments may be very broad [14]. Hard segments and soft segments are phase-separated into hard domain and soft domain. Hard domain may present different shape, i.e. fibrillar hard domain [15], [16], lamellar hard domain [9], [17], [18], ellipsoid-like hard domain [19] and ribbon-like hard domain [20], [21], [22]. Hard domains may form the superstructure [23]. Morphology and properties of TPU can be altered by tuning the content of hard segments [24], [25], [26] and the preparation condition [27].

Physical dimension of hard and soft phases of TPU [28], [29] fits in the accessible range of small-angle X-ray scattering (SAXS) techniques [19], [27], [28], [30], [31], [32]. The morphology on the nanoscale investigated by SAXS can be traced back to Bonart et al. and Clough et al. [28], [33], [34] SAXS techniques have been successfully applied to monitor the nanostructure evolution of TPU materials [9], [10], [19], [35], [36], [37], [38], [39], [40], [41]. Further phase separation may be induced when the TPU is strained slightly [42]. The hard and soft domains are oriented and stacked along the strain direction followed by formation of microfibrils when the TPU is stretched further to large strain. [15], [28], [43], [44], [45], [46], [47]. During the formation of microfibrils [48], hard domains may be tilted, sheared and extended [49], which is related to the domain shape [9], [16]. The microfibrils are composed of extended soft segments and destructured hard domains whose electron density difference is reduced considerably [15]. The induced microfibrils generally generate strong transverse SAXS streaks. The transverse scattering streaks were generated by the electron density inhomogeneities between vertically oriented microfibrils and lateral soft matrix. These oriented microfibrils may exhibit faint periodic correlation [15], [19], [28]. At high strain, the hard domains are destructured [15], [36], [37], [38], [50] or broken into smaller chunks [9], [16]. Destruction of hard domains is interpreted as the pull-out of the hard segments from hard domains [37], [38], [51], [52], [53], [54], [55].

When the TPU materials are subjected to serving at elevated temperature, especially above the T_g of hard domain, the mobility of hard segment is promoted considerably [25], [26], [56]. The hard segments would reorganize or even crystallize at elevated temperature [57]. The morphology variation at elevated temperature can be originated from the weakened interaction between the hard segments. Namely, the mobility of hard segments is enhanced. So the applied stress can shift the hard segments more easily at elevated temperature. The shifted hard segments would reorganize and form new aggregations.

Generally polyurethanes are not easily crystallized. The hard domain on the atomic scale is of disorder, or of paracrystal in nature [17], [58]. It can crystallize or para-crystallize under special conditions [16], [59]. TPUs synthesized from 4,4′-methylene diphenyl diisocyanate (MDI) and 1,4-butanediol (BD) acting as hard segment are investigated vastly [7], [27], [28], [35], [60], [61], [62], [63], even at compression [40]. The MDI/BD segments can crystallize when TPUs are deformed and annealed at the temperature above T_g (70–90 °C) [27], [64] of MDI/BD domain for a very long time [65], [66], [67], [68]. Polymorphic structure was also found [7], [60], [63], [69] which affects the properties of TPU [7], [28], [34], [59], [60], [65], [66], [67], [68], [69].

Hard domains acting as the physical cross-linkers of the system determine the properties of TPU [35], [70]. Weakened internal interaction of hard segments at elevated temperature facilitates the reorganization of hard segments. Therefore, injection-molded TPU parts are annealed at elevated temperature for tens of hours to accelerate the phase separation and to enhance the material strength. During the annealing, hard segments migrate from the mixed matrix to the hard-segment-rich aggregations or clusters. When the TPU is subjected to deformation at elevated temperature above the T_g of hard domain, reorganization of the hard segments in the force field should be very different from reorganization at the temperature lower than the T_g of hard domain. How the hard segments are disorganized and reorganized at elevated temperature under deformation has not been studied broadly.

In our present study, morphological evolution of TPUs with different HSCs subjected to tensile deformation at 100 °C was monitored by in-situ WAXS and SAXS techniques. Herein, deformation temperature is selected elaborately. When the TPU part is injection-molded, the phase separation is not complete. Annealing at 100 °C for several hours is employed for improving the phase separation in application. 100 °C is also higher than the T_g of hard domain. It is also higher than the melting temperature of the possible strain-induced crystallization of polytetrahydrofuran (PTHF) soft segment. The influence of crystallization of PTHF can be avoided. The degradation at 100 °C can be negligible as well. Another factor is that there will be no more obvious annealing effects during the measurement. Atomic packing of MDI/BD segments and arrangement of hard domains and soft domains was studied by in-situ WAXS and SAXS respectively. From the WAXS data, the orientation and variation of atomic conformation was qualitatively discussed. From the SAXS data, the domain arrangement and their size evaluation were extracted and analyzed quantitatively. How the morphology of TPUs with different HSCs evolves at elevated temperature is discussed.