Global and Local Views of the Glass Transition in Mixtures

Experimental evidence for immiscibility of enantiomeric polymers: Phase separation of high-molecular-weight poly(ʟ-lactide)/poly(ᴅ-lactide) blends and its impact on hindering stereocomplex crystallization

Although polymers tend not to mix, it remains challenging to characterize the immiscibility of enantiomeric poly(ʟ-lactide) (PLLA) and poly(ᴅ-lactide) (PDLA), particularly with equivalent and high molecular weight (high MW), which frustratingly disfavors the exclusive stereocomplexation. By introducing a random copolymer (PLC) of ʟ-lactide and caprolactone to form binary blends with PLLA and PDLA, the phase behavior of high-MW PLLA/PDLA blends was investigated mainly by using differential scanning calorimetry (DSC) and atomic force microscopy (AFM). DSC results showed that PLLA/PLC blends exhibited a single glass transition temperature (Tg), which depended on the blending ratio and precisely corresponded with the theoretical values calculated from the Fox equation. In comparison, PDLA/PLC blends showed composition-dependent heat-capacity increment at two unchanged Tg values of pure PLC and PDLA. AFM observation revealed that PLC is completely miscible with PLLA at high MW but is immiscible with PDLA, logically suggesting immiscibility of high-MW PLLA and PDLA. Moreover, AFM results demonstrated that high-MW PLLA/PDLA blends exhibited spherical droplets in asymmetric blends and bicontinuous interpenetrating worm-like patterns in symmetric counterparts, showing distinct and well-defined interfaces, confirming the microphase separation. Additionally, different MWs fundamentally led to significant differences in miscibility, which consequently affected the crystallization behaviors of PLLA/PDLA blends. This work provides evidence for (im)miscibility and its crucial impact on the crystallization of PLLA/PDLA blends and has important implications for understanding the stereocomplexation of polymers.

Disentangling the Calorimetric Glass-Transition Trace in Polymer/Oligomer Mixtures from the Modeling of Dielectric Relaxation and the Input of Small-Angle Neutron Scattering

We have disentangled the contributions to the glass transition as observed by differential scanning calorimetry (DSC) on simplified systems of industrial interest consisting of blends of styrene–butadiene rubber (SBR) and polystyrene (PS) oligomer. To do this, we have started from a model previously proposed to describe the effects of blending on the equilibrium dynamics of the α-relaxation as monitored by broadband dielectric spectroscopy (BDS). This model is based on the combination of self-concentration and thermally driven concentration fluctuations (TCFs). Considering the direct insight of small-angle neutron scattering on TCFs, blending effects on the α-relaxation can be fully accounted for by using only three free parameters: the self-concentration of the components φ_self^SBR and φ_self^PS) and the relevant length scale of segmental relaxation, 2R_c. Their values were determined from the analysis of the BDS results on these samples, being that obtained for 2R_c ≈ 25Å in the range usually reported for this magnitude in glass-forming systems. Using a similar approach, the distinct contributions to the DSC experiments were evaluated by imposing the dynamical information deduced from BDS and connecting the component segmental dynamics in the blend above the glass-transition temperature T_g (at equilibrium) and the way the equilibrium is lost when cooling toward the glassy state. This connection was made through the α-relaxation characteristic time of each component at T_g, τ_g. The agreement of such constructed curves with the experimental DSC results is excellent just assuming that τ_g is not affected by blending.

Manipulating Conjugated Polymer Backbone Dynamics through Controlled Thermal Cleavage of Alkyl Sidechains

The morphological stability of an organic photovoltaic (OPV) device is greatly affected by the dynamics of donors and acceptors occurring near the device's high operational temperature. These dynamics can be quantified by the glass transition temperature (T_g ) of conjugated polymers. Because flexible side chains possess much faster dynamics, the cleavage of the flexible alkyl side chains will reduce chain dynamics, leading to a higher T_g . In this work, we systematically study the T_g for conjugated polymers with controlled sidechain cleavage. Isothermal annealing of polythiophenes featuring thermally-cleavable side chains at 140 °C, a temperature that is below the melting point of polymers, was found to remove more than 95% of alkyl sidechains in 24 hours, and raise the backbone T_g from 23 to 75 °C. Coarse grain molecular dynamics simulations were used to understand the T_g dependence on side chain cleavage. X-ray scattering indicates the relative degree of crystallization remains constant over the course of isothermal annealing. The effective conjugation length is not influenced by thermal cleavage; however, the density of chromophore is doubled after the complete removal of alkyl side chains. The combined effect of enhancing T_g and conserving crystalline structures during the thermal cleavage process could provide a pathway to improving the stability of optoelectronic properties in future OPV devices. This article is protected by copyright. All rights reserved.

Rationalizing the Composition Dependence of Glass Transition Temperatures in Amorphous Polymer/POSS Composites

We report that the fractions of "bonded" or "unbonded" monomers at a filler interface dictate the composition dependence of the glass transition temperatures (T_g) of polyhedral oligomeric silsesquioxane (POSS)-containing nanocomposites. T_g is arguably the single most important material property; however, predicting T_g in nanocomposites is often challenging because of confounding interfacial effects. To this end, we design a model nanocomposite to systematically study T_g of nanocomposites by leveraging the "all-interfacial" nature of ultrasmall POSS fillers loaded into random copolymers of styrene and 2-vinylpyridine (2VP). The amine-functionalized POSS forms hydrogen bonds only with 2VP, which behaves as a "bonded" monomer. The influence of copolymer composition and POSS loading on the T_g of this model composite is successfully explained by a Fox equation framework. This model also captures the T_g increase of other POSS-based polymer composites and potentially directs the future design of nanocomposite materials with tailored T_g.

Poly(etheretherketone)/Poly(ethersulfone) Blends with Phenolphthalein: Miscibility, Thermomechanical Properties, Crystallization and Morphology

Polyetheretherketone (PEEK)/polyethersulfone (PES) blends are initially not miscible, except when the blends are prepared by solvent mixing. We propose a route to elaborate PEEK/PES blends with partial miscibility by melt mixing at 375 °C with phenolphthalein. The miscibility of blends has been examined using differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMTA). When adding phenolphthalein to PEEK/PES blends, the glass transitions are shifted inward as an indication of miscibility. We suggest that phenolphthalein acts as a compatibilizer by creating cardo side groups on PEEK and PES chains by nucleophilic substitution in the melted state, although this condensation reaction was reported only in the solvent until now. In addition, phenolphthalein acts as a plasticizer for PES by decreasing its glass transition. As a consequence, the PEEK phase is softened which favors the crystallization as the increase of crystalline rate. Due to aromatic moieties in phenolphthalein, the storage modulus of blends in the glassy region is kept identical to pure PEEK. The morphological analysis by SEM pictures displays nano- to microsized PES spherical domains in the PEEK matrix with improved PEEK/PES interfacial adhesion.