In this work linear polydimethylsiloxane (PDMS) of five low molecular weights, Wm ∼2–8 kg/mol, i.e. mainly below the entanglement threshold, is studied employing differential scanning calorimetry ...(DSC), thermally stimulated depolarization currents (TSDC), a special dielectric technique in the temperature domain, and broadband dielectric spectroscopy (BDS). It is shown mainly via raw data that the crystallization and glass transition for the larger Wms follow expected trends, however, for Wm ∼2 k (corresponding to ∼22 monomers/chain) crystallization is severely suppressed and slower. Moreover, it is demonstrated that upon isothermal annealing the crystalline fraction (CF) is maximized for all Wms with the exception of the lower one. For the latter, results indicate that during the annealing only the first stages of crystallization (nucleation, ordering of the polymer around the nuclei) take place, while the main crystallization event (formation of crystals) is recorded only upon heating, via the cold crystallization process. The suppressed crystallizability originates from the retarded nucleation. In particular for the lower Wm, the presence of large numbers of chain ends should be at the origins of hindered short range ordering of the polymer chains, as well as in relation to the difficult formation of physical crosslinks. At the same time, in TSDC and BDS uniquely for Wm ∼2 k upon annealing and CF∼0, next to the relaxation peaks related with the glass transition of the bulk-like polymer (α and αc), an additional quite strong peak (αnew) is recorded at higher temperatures. The overall characteristics of αnew relaxation resemble those of PDMS chains constrained and ordered in the interfacial layer in amorphous silica/PDMS nanocomposites (αint relaxation). Thus, combing the overall results with results from the literature, it is proposed that αnew is a direct recoding of retarded dynamics of the polymer chains being ordered around the crystallization nuclei or around extremely small crystals.
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•DSC and dielectric spectroscopy study in PDMS of low MW, below the entanglement threshold.•Severely suppressed crystallization (CF∼0) for the lower MW∼2 k, due to retarded nucleation.•Upon annealing and CF∼0, a new strong nucleation-related dielectric peak (αnew) is recorded.•αnew arises from polymer chains ordered around crystallization nuclei and/or quite small crystals.
Water at interfaces is considered present and important within almost all physical processes. Interfacial water shows nonbulk properties, however, its structure/organization and dynamics are on a ...still open debate. In parallel to that, the interfacial polymer in nanocomposites exhibits modified structure and dynamics, the latter characteristic being widely considered responsible for the improved nanocomposite properties. One of the most suitable tools for studying dynamics of a variety of small molecules and macromolecules is broadband dielectric spectroscopy (BDS), mainly due to its high resolving power. BDS is employed here in combination with isothermal hydration and has proved quite illuminating in recording for the first time striking similarities in the dynamics behavior between water molecules and flexible polydimethylsiloxane macromolecules, when the latter molecules are individually adsorbed physically on the attractive surfaces of metal oxide nanoparticle. The qualitative similarities between water and polymer interfacial dynamics, for example, in their exhibiting time-scale and the elevation of the respective activation energy upon increasing of interfacial molecules fraction, lay most probably at the origins of the same main surface-adsorbed molecule interaction, i.e.. the hydrogen bonding. Whereas, the quantitative differences in molecular dynamics, e.g., slower, more energetically demanding, and cooperative interfacial dynamics for the polymer than for water, arise from the different scale of the two molecular motions (<1 nm for water and ≥1 nm for the polymer) and/or from alternations in interfacial layer density.
The fraction of the polymer located at the nanoparticle–polymer interface in polymer nanocomposites (PNCs) interfacial rigid amorphous fraction (RAFfiller) is under extensive investigation as it is ...generally considered responsible for the modified properties of PNCs as compared to the neat polymers. In semicrystalline PNCs, an additional interfacial polymer exists in the vicinity of crystals, that is, the RAFcrystal, with the properties of the latter (structure and dynamics) being also under investigation. While the presence of nanoparticles and/or crystals themselves can be directly correlated with the modified PNC properties (e.g., mechanical performance), the respective direct effects of the RAFs have been rarely demonstrated. In a recent work, we studied RAF in PNCs based on two different polymers, amorphous stryrene butadiene rubber, wherein RAF = RAFfiller, and semicrystalline low-density polyethylene wherein RAF ≈ RAFcrystal, filled with carbon nanotubes (CNTs), and we revealed quite systematic dependences of thermal diffusivity α on the amount of RAF at each matrix. Strikingly, the α(RAF) trends were diverse for the two matrices, namely, RAFcrystal, which facilitates heat transport, and RAFfiller, which hinders heat transport. In the present work, we checked the latter α(RAF) trends in different PNCs based however on the same polylactide matrix at the amorphous and semicrystalline states, filled with small amounts of thermally conductive CNTs and graphene oxide platelets. Results for α(RAF) here were interestingly found similar with those in the previous study. These findings supply additional evidence for the different structural characteristics exhibited by the two types of RAF, a still open debate in the literature; namely, polymer chains exhibit higher level of ordering in RAFcrystal as compared to RAFfiller. Finally, we propose that RAFfiller can be used as a measure of the phonon’s scattering in PNCs, whereas RAFcrystal in conjunction with bulk crystallites forms additional thermal diffusion paths.
Polymer composites based on poly(lactic acid) (PLA) and three fillers, graphene oxide (GO), silica nanoparticles, and a nanohybrid consisting of silica doped GO, were prepared and investigated. A ...combination of complimentary techniques, namely thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), polarized light microscopy (PLM), differential scanning calorimetry (DSC), and broadband dielectric spectroscopy (BDS), were employed to study the polymer-filler interaction, crystallization and molecular mobility. For the low filler content of 1 wt%, all inclusions act as nucleating agents, increasing the rate of PLA crystallization. Among all fillers, including the mixture of pristine silica and GO, the nanohybrid imposes the stronger effects on crystallization rate, the latter being highly desirable in industrial processing of PLA. The crystalline fraction (∼30%) was found to be the dominant factor in molecular mobility, is agreement with previous investigations on PLA based systems. Based on the degree of polymer/filler interaction (FTIR) and the suppression in calorimetric/dielectric strength of the glass transition (DSC/BDS), the interfacial rigid amorphous fraction (RAF) could be estimated. Interestingly, RAF exhibits its largest value for the composite based on the hybrid filler, in qualitative agreement between the three techniques. The latter fraction, representative of the polymer adsorption onto the nanofillers, is considered important within many applications (thin polymeric films, polymer nanocomposites, etc), furthermore, it is widely believed that RAF dominates the observed improvements of the properties in the nanocomposites.
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•Hybrid GO-nanosilica particles as well as respective PLA nanocomposites were prepared and studied.•The materials were characterized via FTIR, XRD, PLM, TGA, DSC and BDS techniques.•The effects or the hybrid particles are more pronounced than the individual GO and nanosilica.•The Hybrid particles at only 1 wt% increase the PLA adsorption and the polymer crystallization rate.
In this article, we synthesize five poly(n-alkylene succinate)s, PnASs, with n = 2, 4, 6, 8, and 10 via multi-step polycondensation methods. Next, we comparatively investigate these renewable and ...biobased polyesters from the points of view of structure, crystallinity, and molecular mobility, employing 1H nuclear magnetic resonance spectroscopy, size-exclusion chromatography, viscometry, X-ray diffraction, differential scanning calorimetry (DSC, conventional and temperature modulation modes), polarized optical microscopy (POM), and broadband dielectric relaxation spectroscopy (BDS). Next to the successful synthesis of the materials, we evaluate the characteristics of crystallization (temperature and fraction); moreover, we explore for the first time, on the same type of succinic polyesters, the impact of n on the structure memory related to crystal nucleation as well as the changes in the semicrystalline morphology. We demonstrate that the structure/crystal memory is stronger for the lower n (shorter alkylene succinate monomers) because of more chain–chain associations, the result being independent from the overall length of the polymer chain (molar mass, M n 13–80 kg/mass). The crystalline fraction (CF ∼ 12–34%) increases with n, also independently from M n; however, the chain length affects directly the nucleation rate as T c increases with M n. The direct effects of n, in the inter-/intrachain interactions, as well as the indirect ones, on the CF and distribution of crystallites were found to be responsible for the alternations in the static glass transition temperature in DSC (lowering of T g with n) and the dynamic glass transition (α, αc relaxations in BDS). For the sum of these PnASs, the molecular dynamics mapping is shown here, also for the first time. With increasing n, segmental dynamics accelerates, whereas, interestingly, the cooperativity drops (elimination for n = 10). Comparing these results with the recorded alternations in the semicrystalline morphology (POM), we conclude spatial confinement to be imposed on the mobile amorphous polymer by the tightly distributed crystallites when n increases. Overall, these data provide proofs for the potential for tuning of the final polymer properties connected with crystallization (mechanical performance and permeability), envisaging future biomedical, packaging, and other application for these PnASs.
An amphiphilic PEO-b-PPO-b-PEO copolymer (poloxamer) is used as the soft phase of a biocompatible polyurethane matrix. The molecular and charge transport dynamics of the polyurethanes and the ...copolymer is studied in detail by broadband dielectric spectroscopy and differential scanning calorimetry. Various fitting techniques and formalisms are used for the extraction of data from the obtained dielectric spectra. As a result of incomplete phase separation, the glass transition of the polyurethanes occurs 10 K higher than that of the copolymer. The dynamic glass transition of the copolymer (α relaxation) is accompanied by a normal mode relaxation. When the poloxamer is constrained as part of the polyurethane chain, a process slower than α is observed and is likely associated with the normal mode relaxation despite a very significant spectral broadening. The dynamics of the α relaxation and this process does not depend significantly on the soft:hard segment ratio suggesting only minor differences in the consistency of the soft phases. The charge transport mechanism is similar to that of the neat copolymer for high soft segment contents, whereas for the 50 wt% material it changes, presumably as a result of a radical change in morphology.
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•A poloxamer is incorporated in a polyurethane as soft phase.•Its dynamics in free and bound state is studied by dielectric spectroscopy.•Plurality of relaxations is observed in the polyurethane and a detailed analysis is conducted.•Normal mode relaxation is observed in the poloxamer.•Normal mode is likely retained in the polyurethane.
The molecular dynamics of polystyrene (PS) single-chain nanoparticles (SCNPs) with variable molecular weights and irreversible intramolecular cross-linking densities was studied, by employing ...broadband dielectric spectroscopy, supplemented by calorimetry (differential scanning calorimetry, DSC), in the melt. Regarding segmental dynamics, the α relaxation process was found to be retarded for low and moderate cross-linker fractions (CrFs), in agreement with the shift of the corresponding calorimetric glass transitions. Intriguingly, our data indicate that the dynamical behavior of SCNPs is strongly affected by the macromolecular characteristic of the PS precursors, that is, molecular weight (M n) and CrF, as a secondary, Arrhenius-like relaxation process, β*, appears in SCNPs with CrFs larger than a critical value. Our measurements reveal a transition from a strongly heterogeneous structure in loose SCNPs to a homogeneously compact structure in rigid SCNPs with increasing CrF. We propose a dynamic “phase diagram” of loose, dense, or rigid SCNPs, depending on the CrF and M n of the precursor.
In this article we present results on the glass transition, crystallization and molecular dynamics in relatively novel oligomers, oligo-ethylene glycol methacrylate (OEGMA), with short and long ...chains, as well as in the corresponding nanostructured comb-like polymers (POEGMA, short and long), the latter being prepared
via
the RAFT polymerization process. For the investigation we employed conventional and temperature modulated differential scanning calorimetry in combination with high resolving power dielectric spectroscopy techniques, broadband dielectric relaxation spectroscopy (BDS) and thermally stimulated depolarization currents (TSDC). Under ambient conditions short OEGMA (475 g mol
−1
, ∼4 nm in length) exhibits a remarkable low glass transition temperature,
T
g
, of −91 °C, crystallization temperature
T
c
= −24 °C and a significant crystalline fraction, CF, of ∼30%. When doubling the number of monomers (OEGMA-long, 950 g mol
−1
, chain length ∼8 nm) the
T
g
increases by about 20 K and CF increases to ∼53%, whereas, the
T
c
migrates to a room-like temperature of 19 °C. Upon formation of comb-like POEGMA structures the grafted OEGMA short chains, strikingly, are not able to crystallize, while in POEGMA-long the crystallization behaviour changes significantly as compared to OEGMA. Our results indicate that in the comb-like architecture the chain diffusion of the amorphous fractions is also strongly affected. The semicrystalline systems exhibit significant melt memory effects, this being stronger in the comb-like architecture. It is shown that these effects are related to the inter- and intra-chain interactions of the crystallizable chains. The dielectric techniques allowed the molecular dynamics mapping of these new systems from the linear oligomers to POEGMAs for the first time. BDS and TSDC detected various dynamics processes, in particular, the local polymer dynamics (
γ
process) to be sensitive to the
T
g
, local dynamics triggered in the hydrophilic chain segments by water traces (
β
), as well as the segmental dynamics (
α
) related to glass transition. Interestingly, both the short and long linear OEGMAs exhibit an additional relaxation process that resembles the Normal-Mode process appearing in polyethers. In the corresponding POEGMAs this process could not be resolved, this being an effect of the one-side grafted chain on the comb backbone. The revealed variations in molecular mobility and crystallization behavior suggest the potentially manipulable diffusion of small molecules throughout the polymer volume,
via
both the molecular architecture as well as the thermal treatment. This ability is extremely useful for these novel materials, envisaging their future applications in biomedicine (drug encapsulation).
Synthesis, crystallization, and molecular dynamics investigation on oligo-ethylene glycol methacrylate (OEGMA) of short and long chains, in the linear and nanostructured comb-like forms.
We investigate the thermal transitions and molecular mobility in new nanocomposites of biobased poly(ethylene furanoate) (PEF), by calorimetry and dielectric spectroscopy, supplemented by X-ray ...diffraction, Fourier transform infra-red spectroscopy and polarized light microscopy. The emphasis is placed on the facilitation of the crystallization of PEF, which is in general low and slow due to structural limitations that result in poor nucleation. Tuning of the crystalline fraction (CF) and semicrystalline morphology are important for optimization of the mechanical performance and manipulation of the permeation of small molecules (
e.g.
, in packaging applications). The nucleation and CF are successfully improved here by the
in situ
filling of PEF with 0.5–2.5 wt% of carbon nanotubes (CNTs) and graphene nanoplatelets (GNPs). The improvements are discussed in connection with weak or absent interfacial polymer–filler interactions. CNTs were found to be more effective in facilitating crystallization, as compared with GNPs, possibly due to their larger aspect ratio. The segmental dynamics of PEF are both accelerated and decelerated by the addition of GNP and CNT, respectively, with complex phenomena contributing to the effects, namely, nucleation, changes in molar mass and changes in the free volume. The molecular mobility of PEF is moderately affected ‘directly’ by the particles, whereas stronger effects are induced by crystallization (an indirect effect) and, furthermore, by the increase in the length of alkylene sequences on the chain. Local dynamics exhibit time scale disturbances when the temperature approaches that of the glass transition, which is proposed here to be a common characteristic in the case of mobilities originating from the polymer backbone for these as well as different polyesters. Despite the weak effects on molecular mobility, the role of the fillers as nucleating agents seems to be further exploitable in the frame of envisaged applications, as the use of such fillers in combination with thermal treatment offer possibilities for manipulating the semicrystalline morphology, ion transport and, subsequently, permeation of small molecules.
Biodegradable polymers offer a promising alternative to the global plastic problems and especially in the last decade, to the microplastics problems. For the first time, samples of poly(butylene ...succinate) (PBSu) biocomposites containing 1, 2.5, and 5 wt% biochar (BC) were prepared by in situ polymerization via the two-stage melt polycondensation procedure. BC was used as a filler for the PBSu to improve its mechanical properties, thermal transitions, and biodegradability. The structure of the synthesized polymers was examined by
H and
C nuclear magnetic resonance (NMR) and X-Ray diffraction (XRD) along with an estimation of the molecular weights, while differential scanning calorimetry (DSC) and light flash analysis (LFA) were also employed to record the thermal transitions and evaluate the thermal conductivity, respectively. It was found that the amount of BC does not affect the molecular weight of PBSu biocomposites. The fine dispersion of BC, as well as the increase in BC content in the polymeric matrix, significantly improves the tensile and impact strengths. The DSC analysis results showed that BC facilitates the crystallization of PBSu biocomposites. Due to the latter, a mild and systematic increase in thermal diffusivity and conductivity was recorded indicating that BC is a conductive material. The molecular mobility of PBSu, local and segmental, does not change significantly in the biocomposites, whereas the BC seems to cause an increase in the overall dielectric permittivity. Finally, it was found that the enzymatic hydrolysis degradation rate of biocomposites increased with the increasing BC content.
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