Effect of interfacial interactions on static and dynamic behavior of hyperbranched polymers: Comparison between different layered nanoadditives
Graphical abstract
Introduction
Polymer nanohybrids, which consist of inorganic, metal/metal oxide or carbon-based nanoadditives (e.g., graphene, nanotubes, silicon dioxide, nanoparticles, clays, etc.) finely dispersed, in most of the times, within a polymer matrix, exhibit enhanced and often innovative physicochemical properties compared to their conventionally filled counterparts [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10]]. These optimized properties are ascribed to the size of the additives, which is in the range of nm, that leads to a significant increase of the interfacial area, thus, creating a large proportion of “interfacial” polymer chains, that possess different properties than the corresponding ones of the bulk polymer even at a low content [[11], [12], [13]]. Understanding the correlation between the physicochemical features of the nanoadditives and of the polymer and the final properties of the nanohybrid is of pronounced significance for the design of novel materials with specific functionalities for advanced applications [[14], [15], [16], [17], [18], [19]]. Along these lines, the way polymers crystallize close to surfaces and/or under confinement and how one can control their crystallization has attracted scientific interest because it is a fundamental problem and it is very important for technological applications [[20], [21], [22], [23], [24]]. At the same time, the polymer chain conformations are modified close to inorganic surfaces and/or under confinement, as well [20,25,26]. Although the investigation of the structure of the nanohybrid materials and the optimization of their properties have attracted most of the scientific interest, the study of polymer dynamics in the presence of nanoadditives or under confinement has received less attention by the scientific community [[27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37]].
An especially interesting case among the various nanoadditives is the case of layered additives. In this case, based on the interactions between the polymer chains and the inorganic surfaces, three different structures can be obtained [38]: the phase separated, where there are unfavorable interactions between the chains and the inorganic fillers and each component resides in its own phase, the intercalated, where the polymer can diffuse between the inorganic layers forming a well ordered multilayer with a repeat distance of a few nanometers, and the exfoliated one, where the favorable interactions lead to the destruction of the layered structure of the inorganic material and to the dispersion of individual inorganic platelets within the polymeric matrix. Although the optimal properties of the nanocomposites are anticipated for exfoliated [[39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50]] or intercalated [20,35,37,[51], [52], [53], [54], [55], [56]] structures because of the good interfacial interactions, the high aspect ratio and the large surface area, the intercalated structure offers the capability to investigate static and dynamic properties of macromolecules in nano-confinement, using, however, conventional analytic techniques on macroscopic samples [57,58]. Two of the most interesting layered materials are natural layered silicates and graphite oxide. Despite the hydrophilic character of both materials, which renders them suitable for mixing with polar polymers, their chemical composition as well as the structure of their surfaces are very different and, thus, different interactions and different affinity with certain polymers are anticipated. Layered silicates, like for example sodium montmorillonite (Na+-MMT), have been often utilized in the past to prepare polymer nanocomposites and study their behavior. An intercalated structure was observed when Na+-MMT was mixed with various hydrophilic linear polymers, like poly (ethylene oxide) [20], PEO, poly (hexa ethylene glycol methacrylate) [32], PHEGMA, biobased polyesters [37] or hydrophilic dendritic and star polymers [25,35]; polymer films with quantized thickness in the range of 0.4–1.2 nm were formed within the galleries of the inorganic material depending on the composition. Moreover, the effect of the additive as well as of the spatial confinement on the polymer morphology, chain conformations and dynamics was investigated. Thermal transitions like glass transition or crystallization were found to be suppressed under confinement, with the degree of crystallinity exhibiting a sharp decrease as a function of composition. Moreover, a significant increase of the gauche conformations in the melt indicated that the confined chains were more disordered and liquid-like [20].
The situation is even more complicated concerning polymer dynamics, where both faster or slower segmental dynamics have been observed compared to that of the bulk polymer, with either Arrhenius or Vogel-Fulcher-Tammann (VFT) temperature dependencies of the relaxation time depending on the polymer molecular weight (i.e., degree of confinement) or on the functional groups of the molecule [29,33,35,37]. Moreover, sub-glass transition temperature processes possess similar or faster relaxation times depending on the ability of the chain to form intra- and/or inter-molecular hydrogen bonds [35].
Many graphitic materials, like graphite itself or graphite oxide, possess a multilayered structure as well. In recent years, graphitic materials are of great interest due to their unique structural and morphological features, their relatively easy chemical modification as well as their excellent electrical, thermal and mechanical properties. Especially polymer/graphitic material nanocomposites have found plethora of applications for energy materials, solar cells, water splitting, biomedical applications, environmental and catalytic technologies as well as for electromagnetic shielding [[59], [60], [61], [62]]. During the last decade, there have been extensive research efforts on the preparation and investigation of the properties of GO, although graphite oxide was synthesized many decades before by the oxidation of graphite [63,64]; thus, a new additive that can exhibit tailored surface properties by functionalization is obtained. Moreover, graphite oxide can be mixed with hydrophilic polymers in order to obtain nanocomposites with either exfoliated or intercalated structures. Although it is mainly graphene or reduced graphene oxide that have been utilized for the preparation of polymer nanocomposites, there are a few works that utilized graphite oxide [[65], [66], [67], [68], [69], [70]]; nevertheless, in the majority of them, just a few percent of additive was used, which precluded the investigation of the polymer structure and dynamics under confinement [34,[71], [72], [73]].
On the other hand, polymer architecture is of great importance in polymer science since it affects physical properties including melt and solution viscosity, glass transition temperature, solubility in various solvents as well as the size of the chains in solution. A relatively new class of non-linear polymers that have attracted the scientific interest are the dendritic ones because of their exceptional characteristics like low viscosity, high density, and the existence of a large number of functional end-groups that allows them to be utilized in many applications [74,75]. They can be divided in dendrimers, which exhibit well-ordered, symmetric, tree-like structure, with arms organized into perfect generations with well-defined molecular weight, and in hyperbranched polymers, HBPs, which have a varying number of arm points, an asymmetric structure and a certain degree of polydispersity. HBPs show similar features with the dendritic macromolecules, in conjunction with the additional benefit of their cost-effective synthesis, feasible by one-pot synthetic methods, and not requiring purification procedures [76,77]. Hyperbranched polyesters [78] are an interesting class of HBPs, which have been investigated mostly regarding their synthesis and their molecular characterization [[79], [80], [81]] and less regarding their structure and their ability for hydrogen-bond networks formation [82], their structure-property relationship [83] and their dynamics [35,[84], [85], [86], [87], [88]].
For hyperbranched polymer/layered silicate nanocomposites, there is only a small number of works, which showed that, for hybrids with low amount of additive, an exfoliated structure is obtained, whereas, for high amounts of layered silicate inclusions, HBPs promote intercalated structures [89]; these findings were attributed to the polymer globular conformations and its high number of functional end-groups. A change in the rheological response of the nanocomposites is obtained, whereas their stiffness, strength and strain at break are found to increase especially in the case when an exfoliated structure is identified. For intercalated hyperbranched polyester nanocomposites, both differences and similarities were observed in the polymer dynamics when compared to the bulk polymer material depending on the relaxation process, its characteristic length scale and the ability to form hydrogen bonds [35,87]. Hyperbranched polyesters have been utilized for the functionalization of graphite oxide surfaces as well [90]; nevertheless, there are no works that investigate the structure, properties and dynamics of the polymers themselves when confined between the GO surfaces. Moreover, there are no works that attempt to compare the static and dynamic behavior of hyperbranched polyesters when they are intercalated in different types of layered materials like layered silicates and graphite oxide.
In this paper, a hyperbranched polyester amide, Hybrane®, is mixed with natural hydrophilic montmorillonite, Na+-MMT, and with graphite oxide in order to compare the effect of the interfacial interactions on the obtained structure and on the polymer thermal properties and relaxation dynamics. Intercalated structures are obtained in both cases; nevertheless, differences in the interlayer distances are observed because of the different polymer/surface interactions. Moreover, thermal reduction of GO in the presence of the polymer shows that the structure of the nanocomposite changes and a further expansion and/or even a full exfoliation of the GO layers is observed; the reduction temperature, where deoxygenation occurs, is found lower in the presence of the polymer than the respective one for the bare GO. In both cases, the glass transition of the intercalated chains is fully suppressed. The different surface interactions affect the polymer dynamics as well. The two sub-Tg secondary relaxation processes are observed in both nanocomposites with characteristic times possessing different temperature dependencies compared to the respective of the neat polymer whereas there is a process related to a branch motion that is observed only in the case of the GO nanocomposite. On the other hand, the segmental dynamics of the confined chains is observed either faster or slower than the one of Hybrane polymer; nevertheless in both cases it shows an Arrhenius temperature dependence.
Section snippets
Materials
Hybrane® S 1200, a hyperbranched poly (ester amide) [78] with a number-average molecular weight Mn = 1200 g/mol, was kindly provided by DSM. A probable chemical structure of the molecule, demonstrating its imperfect structure, is shown in Scheme 1. Hybrane possesses a number of hydroxyl and carboxyl groups that are able to form both intra- and inter-molecular hydrogen bonds. Hybrane is amorphous, with a glass transition temperature, Tg, of 315–320 K (according to its Safety Data sheet).
Structural behavior of hybrane nanocomposites
Fig. 1a shows X-ray diffraction measurements of graphite oxide, GO, to verify the quality of its layered structure. Initial measurement showed a strong peak at 2θ = 10.7° corresponding to an interlayer distance of 0.82 nm, the narrow thickness of which verifies the significant coherence of GO structure with an average of ~15 GO layers/particle (according to the Scherrer equation). Please note that the specific measurement was performed following drying of the GO in vacuum at room temperature
Conclusions
The static and dynamic behavior of two nanocomposites, comprised of a hyperbranched polyester amide and two different layered hydrophilic additives, was investigated to probe the effect of the different polymer/surface interactions. One of the additives was sodium montmorillonite, Na+-MMT, and the other one was graphite oxide, GO. In both cases, an intercalated structure was obtained and, for the compositions investigated, all chains are anticipated to reside within the interlayer galleries.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This research has been partially supported by the project “National Research Infrastructure on nanotechnology, advanced materials and micro/nanoelectronics” (MIS 5002772) implemented under the “Action for the Strategic Development on the Research and Technological Sector”, funded by the Operational Programme “Competitiveness, Entrepreneurship and Innovation” (NSRF 2014–2020) and co-financed by Greece and the European Union (European Regional Development Fund). The authors acknowledge the
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