Differential scanning calorimetry liquid-crystalline polymers

Ferrocene-containing liquid crystalline polymers 30 have been reported from the solution polymerization of l,T-bis(chlorocarbonyl)ferrocene, terephthaloyl chloride, and methylhydroquinone in refluxing dichloromethane [38], as indicated in Scheme 10-11. With one exception, these ferrocene containing copolyesters were reported to have birefringent melts. The presence of liquid crystallinity was verified by differential scanning calorimetry (DSC), polarized light microscopy, and X-ray diffraction studies. 

Characterization. The liquid crystalline properties of the side-chain monomers (III) and polymers (I) have been studied by Differential Scanning Calorimetry (DSC), Polarized Optical Microscopy (POM) and X-ray diffraction. The thermal transition data and phase types for all monomers (III) and polymers (I) are summarized in Table HI. A representative DSC scan for the monomer (El) and polymer (p with a four-carbon tail (n=4) and six-carbon flexible spacer (m=6) are shown in Figures 1 and 2 respectively. The first peak at -24°C shown in Figure 1 is the crystal to smectic... 

A complete characterization of liquid crystalline polymers should include at least two aspects the characterization of the molecular structure and that of the condensed state structure. Since the first characterization is nothing more than what is practiced for non-liquid-crystalline polymers, we will restrict the discussion to only a short introduction of methods mostly used in the characterization of the presence and the main types of polymeric liquid crystal phases. The methods include the mostly used polarizing optical microscopy (POM, Section 4.1), differential scanning calorimetry (DSC, Section 4.2) and X-ray diffraction (Section 4.3). The less frequently used methods such as miscibility studies, infrared spectroscopy and NMR spectroscopy will also be discussed briefly (Section 4.4). 

The first set of experiments involved fluorescence resonance energy transfer (FRET) between the naphthalene and pyrene-laheled polymers. A 5 1 mixture of PNIPAM-Py to PNIPAM-Na was used. When assembled in micelles, the pyrene acts as a quencher to the naphthalene, leading to high pyrene fluorescence and low naphthalene fluorescence. When the mixture is added to DMPC (liquid phase) or DSPC (gel phase) vesicles at room temperature, naphthalene fluorescence is increased, while pyrene fluorescence is dramatically decreased. This effect is not seen with the PNIPAM-Py-Na polymer, so the reduction in FRET is not due to the hydrophobic environment. This means that the hydrophobic anchors of the PNIPAM-Py and the PNIPAM-Na likely enter the membrane and the dyes are moved apart from one another. The fact that the anchor appeared to insert into the gel-phase DSPC membrane was somewhat surprising. The authors attribute the effect to defects between crystalline domains in the membrane. To test if the LCST transition still occurs when the polymers are anchored to the membrane, differential scanning calorimetry (DFC) was used. The LCST transition of the PNIPAM-Py/PNIPAM-Na mixture in solution was observed in the DFC ttace. When combined with DSPC or DMPC vesicles, the same peak was observed, indicating that the transition does indeed stiU occur, even in the presence of the lipid. 

Many cellulose derivatives form lyotropic liquid crystals in suitable solvents and several thermotropic cellulose derivatives have been reported (1-3) Cellulosic liquid crystalline systems reported prior to early 1982 have been tabulated (1). Since then, some new substituted cellulosic derivatives which form thermotropic cholesteric phases have been prepared (4), and much effort has been devoted to investigating the previously-reported systems. Anisotropic solutions of cellulose acetate and triacetate in tri-fluoroacetic acid have attracted the attention of several groups. Chiroptical properties (5,6), refractive index (7), phase boundaries (8), nuclear magnetic resonance spectra (9,10) and differential scanning calorimetry (11,12) have been reported for this system. However, trifluoroacetic acid causes degradation of cellulosic polymers this calls into question some of the physical measurements on these mesophases, because time is required for the mesophase solutions to achieve their equilibrium order. Mixtures of trifluoroacetic acid with chlorinated solvents have been employed to minimize this problem (13), and anisotropic solutions of cellulose acetate and triacetate in other solvents have been examined (14,15). The mesophase formed by (hydroxypropyl)cellulose (HPC) in water (16) is stable and easy to handle, and has thus attracted further attention (10,11,17-19), as has the thermotropic mesophase of HPC (20). Detailed studies of mesophase formation and chain rigidity for HPC in dimethyl acetamide (21) and for the benzoic acid ester of HPC in acetone and benzene (22) have been published. Anisotropic solutions of methylol cellulose in dimethyl sulfoxide (23) and of cellulose in dimethyl acetamide/ LiCl (24) were reported. Cellulose tricarbanilate in methyl ethyl ketone forms a liquid crystalline solution (25) with optical properties which are quite distinct from those of previously reported cholesteric cellulosic mesophases (26). 

Differential scanning calorimetry (DSC) analyzes thermal transitions occurring in polymer samples when they are cooled down or heated up under inert atmosphere. Melting and glass transition temperatures can be determined as well as the various transitions in liquid crystalline mesophases. In a typical DSC experiment, two pans are placed on a pair of identically positioned platforms connected to a furnace by a common heat flow path. One pan contains the polymer, the other one is empty (reference pan). Then the two pans are heated up at a specific rate (approx. 10 K min ). The computer guarantees that the two pans heat at exactly the same rate -despite the fact that one pan contains polymer and the other one is empty. Since the polymer sample is extra material, it will take more heat to keep the temperature of the sample pan increasing at the same rate as the reference pan. A plot is created where the difference in heat flow between the sample and reference is plotted as a function of temperature. When there is no phase transition in the polymer, the plot parallels the x-axis, and the heat flow is given in units of heat, q, supplied per unit time, t ... 

Fig. 2 Schematic five types of SMPs depicted as a function of their thermal behavior. Plotted is the heat flow vs temperature as measured in a differential scanning calorimetry (DSC) experiment (a) Cat. A-1, chemically crosslinked tunorphous polymer network (Tirans = 7g) (b) Cat. A-11, chemically ciossfinked semicrystaUine polymer networks (Taims = 7m)> ( ) Cat. B-1, physically crosslinked thermoplastic with Tirans = T (d) Cat. B-11, physically crosslinked thermoplastic (Tams = I m) and (e) liquid crystalline polymer (Tlrans = T -n)...

Liquid crystalline amphiphilic diblock copolymers poly(ethylene oxide)-h/oc -ll-[4-(4-butylphenyl-azo)phenoxy]-undecyl methacrylate, PEOn,-h-PMA(Az) , as shown in Fig. 16, prepared by atom transfer radical polymerization [61], were composed of hydrophilic PEOn, sequences and hydrophobic PMA(Az) , with azobenzene moieties such as mesogen connected by a flexible spacer. The synthesis of such amphiphilic liquid crystal block copolymers has been recently reported [62]. In diblock copolymers PEO ,-h-PMA(Az)n, m and n indicate the degree of polymerization of PEO and PMA(Az) components, respectively. Differential scanning calorimetry (DSC) of PEO ,-f>-PMA(Az)n gives a clear picture of the thermal properties of these liquid crystaUine polymers, as shown in Fig. 17, for PEOn4-h-PMA(Az)2o [58, 61]. 

---