Evaluate the differences between topological structure of low and high molecular compounds. Give examples
By using several different polymers and compounds, organics or inorganic, low-molecular-weightor highmolecular- weight, this study has proven that they can be packed into concentric ring bands, usually circular, but other shapes like hexagons or flower-like petals can also be possible. With these traits taken into consideration, it is difficult to generalize the ring bands in spherulites to a single cause of lamellae twist/spiral. Instead, exposure of lamellae underneath the top layers beyond thin films becomes essential for shedding new light on all these intricately complex issues. The novel approaches in this study circumvent such limitation by interior dissection of PEA, which clearly reveals that no continuous spiraling twist, as the cross sections show a corrugated-board structure with layers resembling a peel-able onion, where each radially oriented layer is sandwiched with a tangential layer of lamellae.
Biodegradablepolymers, such as poly(L-lactic acid) (PLLA), poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHB-co- HV), and poly(3-hydroxybutyrate) (PHB), etc., have been shown to exhibit ring-banded spherulites just like many of other arylpolyesters (PPT, POT, PNT, etc.) or aliphatic polyesters (PEA, PBA, etc.). However, most investigators only focused on analyses based on top-surfaces of thin films, and very rarely the inner structures of banded materials were examined. Polymers are known to have chain folding from classical literature and some investigators including Lotz and Cheng argued that chain folding induced surface stresses, which in turn were responsible for crystal twisting. By contrast, smallmolecule compounds, including hippuric acid , phthalic acid , testosterone propionate, and aspirin , etc., do not have chain folding in crystals at all, but the crystals of some of the compounds also twist. Kahr et al, following the initiating arguments of a classical work by Bernaueron many organic compounds using optical microscopy in 1929, have argued that temperature-gradient induced stresses, and other stresses, etc., may cause helice shapes in these smallmolecule crystals. Apparently, the above comparisons between longchain polymers vs. small-molecule compounds clearly hint that crystal twist is a habit that not necessarily is related to or induced by chain folding. A small-molecule compound, phthalic acid (PA), has been extensively investigated by Kahr et al. [2], in revealing ring-banded spherulites by evaporating PA from 20/80 water/ethanol solution at ambient temperature (25°C). This PA compound and its spherulites serve as a useful low-molecule model in comparison to the ring-banded long-chain polymer (PLLA) or aliphatic polyester (PEA), aryl polyester (PDoT) to be dicussed in this work.
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An even more critical issue is that albeit the facts of stress-incduced twisting in single-crystals of either small-molecule compounds or polymers, are there correlations between the ring-banded patterns and the twisting of single crystals. Key point may be in providing direct evidence of whether or not the observed pitch of the twist single crystals is in agreement with the pitch of optical birefringent patterns in ring-banded spherulites. This single most critical evidence (twist pitch of single crystals being in agreement with optical ring interspacing in banded spherulites) appears to be missing in numerous works reported in the literature. Many investigators only focused on analyzing the twisting of single crystals and mechanisms therein, and argued that these twist crystals could be retrived from the ring-banded spherulites. However, these proposed correlations may be an Achilles-knee problem that should be tackled with more careful and deeper analyses, as they may run riskes of the facts that twisted crystals may also be retrived from ringless and non-banded spherulites.
To tackle the complex issues, novel approaches were taken in the work to examine the interiors of crystallized samples that have been prepared into bulk forms. In addition, thickness was varied from very thin to thick to observed changes in crystallized patterns. Top morphology and crystal patterns on the top surface in correlation with the interior crystal lamellae was established. As the surface topology differs among different materials with banding patterns, the inner structures may also differ. Several polymers, mainly synthetic biodegradable polyesters, as well as small-molecules were used as models for comparison and testing for universal behavior in crystals that form ring bands. In addition, different types of ring bands were analyzed and mechanisms exemplified.
13.Colloid-dispersed structural organization of polymers. Evaluate the differences between colloidal structure of low and high molecular compounds.
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14.Discuss methods for the synthesis of well-defined block copolymers via reversible addition fragmentation chain transfer polymerization in the presence of mono- and difunctional RAFT-agents.
The reversible addition fragmentation chain transfer (RAFT) bulk polymerization of a fast propagating monomer (methyl acrylate, MA) has been studied using 1-phenylethyl dithiobenzoate (1-PEDB) and 2-(2-cyanopropyl) dithiobenzoate (CPDB) as RAFT agents at 60 °C. Rate retardation with increasing initial RAFT agent concentrations is common to both 1-PEDB- and CPDB-mediated MA polymerizations and occurs in comparable magnitude. A pronounced inhibition period is observed in 1-PEDB-mediated MA polymerizations, whereas the corresponding CPDB-mediated polymerizations show considerably less inhibition. The cause for this inhibition may either be associated with the leaving group of the initial RAFT agent or with the slow fragmentation of the initial intermediate macroRAFT radical. The present experimental data suggest that slow fragmentation is the probable cause for inhibition. We conclude that the radical intermediate formed by addition of radicals to the initial RAFT agent is different in stability than the macroRAFT radical formed analogously from macroRAFT agent. The inhibition period is effectively reduced by the use of CPDB as the initial RAFT agent in methyl acrylate polymerizations.
Macrocyclic poly(styrene-b-butadiene) (SB) block copolymers were prepared by coupling a living poly(styrene-b-butadiene-b-styrene) (SBS) block copolymer using a living coupling agent, 1,3-bis(1-phenylethylenyl)benzene (DDPE), or a difunctional electrophile, dimethyldichlorosilane. The living poly(styrene-b-butadiene-b-styrene) block copolymer was generated from an addition product of sec-butyllithium and DDPE. A living heteroarmed star block copolymer has been prepared by coupling two moles of monolithium polystyrene with one mole of DDPE followed by reinitiation and polymerization of the butadiene monomer. The dilithium 4-armed star block copolymer was then coupled using dimethyldichlorosilane to form a cyclic polybutadiene with two attached polystyrene branches.
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15. Discuss methods for the synthesis of well-defined block copolymers via atom transfer radical polymerization. Of the several controlled/”living” free radical polymerization techniques, ATRP seems to be the most versatile, being able to polymerize a variety of monomers, method to prepare block and graft copolymers. ATRP enables the synthesis of a wide range of (co)polymers with controlled molecular weight, narrow molecular weight distribution, and range of architectures and functionalities. The synthesis of well-defined AB type diblock copolymers from styrene (S) and methyl acrylate (MA) via ATRP was reported by Schubert and co-workers . Both synthetic routes starting with styrene as first block or methyl acrylate as first block are shown in
Scheme 2. Synthesis of triblock PDMAEMA-b-PCL-b-PDMAEMA copolymer
Huang et al. reported the preparation of amphiphilic triblock PMMA-b-PDMAEMA-bPMMA and poly(methyl acrylate)-b-poly(methyl methacrylate)-b-poly(hydroxyethyl methacrylate) (PMA-b-PMMA-b-PHEMA) copolymer brushes grown from a surface via sequential low-temperature ATRP (Scheme 3)
Scheme 3. Synthesis of triblock copolymer brushes from gold surfaces by sequential ATRP. Recently many studies on the synthesis of block copolymers via ATRP in the literature have been reported due to its advantages. Some of them are listed.
16 Discuss methods for the synthesis of well-defined block copolymers via nitroxide mediated radical polymerization. Compare two basic strategies of NMP initiation.
Graft copolymerization via NMP
Hua et al. prepared polystyrene grafted chitosan [81] (Scheme 25) and poly(sodium 4-styrenesulfonate) grafted chitosan [82] (Scheme 26) via NMP using chitosan-TEMPO macroinitiator.
Schematic presentation of Grafting-through
Synthesis of Chitosan-g-PSt by NMP
Synthesis of Chitosan-g-PSS by NMP
Jaymand reported the synthesis and characterization of novel type poly(4-chloromethyl styrene-graft- 4-vinylpyridine)/TiO2 nanocomposite via NMP [83]. Firstly, poly(4-chloromethyl styrene)/TiO2 nanocomposite carrying TEMPO groups (PCMS-TEMPO/TiO2) was synthesized (Scheme 27) and then the controlled graft copolymerization of 4-vinylpyridine was initiated by PCMS-TEMPO/TiO2 as a macroinitiator (Scheme 28).
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