Observation of Novel Liquid-Crystalline Phase above the Bulk-Melting Temperature

Gautam, Keshav S.; Kumar, Satyendra; Wermeille, D.; Robinson, Doug; Dhinojwala, Ali

doi:10.1103/physrevlett.90.215501

Cited by 22 publications

(36 citation statements)

References 16 publications

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“…The polydispersity (PD) of these polymers was broad (10-20; M n 15 000 g mol ÿ1 ) due to the melt polymerization process. The results were consistent with the narrower PD polymers prepared by transesterification of poly(tert-butyl acrylate) having a PD of 1.12 with an alcohol of appropriate chain length [6]. GIXD experiments were performed on thin films (100 -150 nm) of poly(n-alkyl acrylate)s coated on silicon wafers or glass slides.…”

supporting

confidence: 52%

“…Here n eff is the thickness of the surface ordered phase in units of the number of methylene groups in n-alkane. The value of l in poly(n-alkyl acrylate)s is higher than the corresponding n-alkane because of the additional length of the linker group in the surface ordered phase, as revealed by x-ray reflectivity measurements [6]. In the case of poly(n-alkyl acrylate)s, the x-ray reflectivity fits matched the experimental data only after using the combined thickness of the alkyl part of the side chain and the acrylate linker group.…”

mentioning

confidence: 82%

“…The diffraction pattern did not change upon azimuthal in-plane rotation of the films, indicating that the sample was polycrystalline with random grain orientations in the surface plane. The Bragg rods disappear on further increase in temperature even though the surface remains ordered [6]. The position of this Bragg peak corresponds to an in-plane lattice spacing of 4.2 Å .…”

mentioning

confidence: 92%

“…Interestingly, chemical attachment of alkyl chains using an acrylate linker group to a flexible polymeric backbone [poly(n-alkyl acrylate)s shown in Fig. 1(a)] leads to a dramatic increase in T (9-20 K) [6]. The surfaces of these polymers exhibit a two-step transition from ordered-to-disordered state upon heating.…”

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confidence: 97%

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Partial Crystallinity in Alkyl Side Chain Polymers Dictates Surface Freezing

et al. 2008

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We have studied the structure of a novel crystalline surface monolayer on top of a disordered melt of the same material [poly(n-alkyl acrylate)s] using grazing incidence x-ray diffraction. The grazing incidence x-ray diffraction, surface tension, and bulk latent heat results show that side chains crystallize except the nine methylene units of the alkyl side chains closest to the polymer backbone. The partial crystallinity along with a thicker surface layer, due to the additional length of the linker group, explains why the difference between the surface order-to-disorder transition temperature and bulk melting temperature increases with a decrease in the length of the alkyl side chain.

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confidence: 52%

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confidence: 82%

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confidence: 92%

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confidence: 97%

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Partial Crystallinity in Alkyl Side Chain Polymers Dictates Surface Freezing

et al. 2008

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“…This can be due to only a fraction of side chains crystallizing or only a fraction of methylene units within the side chain participating in the crystallinity. On the basis of our grazing incidence X-ray diffraction results 6 we believe that this is predominantly due to partial crystallinity within the side chains. The same conclusion was also reached for bulk crystallization by Jordan et al 11 This partial involvement of side chain in surface ordered phase implies that (dγ/dT) T<Ts2 for poly(n-alkyl acrylates) is lower than alkanes.…”

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Thermodynamic Study of a Novel Surface Ordered Phase above the Bulk Melting Temperature in Alkyl Side Chain Acrylate Polymers

2005

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Surfaces of most liquids in contact with air have higher entropy and lower melting temperature T s than the bulk. 1 However, materials consisting of linear alkyl chains such as n-alkanes and alcohols exhibit surface freezing, where the surfaces remain ordered 1-3 K above the bulk melting temperature (T m ). 2-4 Interestingly, chemical attachment of linear alkanes to a polymeric backbone (poly(n-alkyl acrylates)) dramatically increases the temperature window where the ordered phase exists (∆T ) (T s -T m ) ≈ 9-20 K). In fact, surface sensitive infrared-visible sum frequency generation spectroscopy and X-ray scattering show the presence of two distinct surface transitions. 5,6 One (T s1 ) associated with crystalline to smectic-like phase and the other (T s2 ) from smectic-like to disordered liquid phase. Why these side chain polymers show enhanced temperature range of surface order in comparison to small molecule alkanes and alcohols is not understood. Intuitively, this could be attributed to lower entropy due to chemical attachment of the side chains to the polymer backbone (loss of positional entropy). Here, we report the first surface tension measurements of side chain acrylate polymers as a function of temperature to measure the thermodynamic parameters of this ordered surface phase. Surprisingly, it is not the positional entropy but partial crystallinity that controls the magnitude of ∆T. These results offer interesting possibilities to control the surface properties by altering the linker groups that attach these hydrocarbon chains to the polymer backbone for applications in the areas of smart adhesives and release coatings. 7,8 Poly(n-alkyl acrylates) were obtained from Landec Corporation and were used as received. The polydispersity of these polymers were broad (10-20) due to melt polymerization process. Measurements on narrower polydisperse samples (≈3) showed similar results. Also the results were consistent with poly(n-alkyl acrylates) prepared by transesterification of poly(tert-butyl acrylate) having polydispersity of 1.12 with an alcohol of an appropriate chain length. 5 Differential scanning calorimetry (DSC) performed at a heating rate of 0.5 K/min showed a sharp melting transition and diffuse freezing transition for side chain length, n ) 16, 18, and 22 (see Supporting Information). The bulk transition temperatures (T m and T f ), enthalpy (in this paper enthalpy is assumed to be equivalent to energy because of negligible contribution of pdV, ∆ b ), and entropy ∆S b of melting are shown in Tables 1 and 2. The suffix b corresponds to bulk. ∆ b and ∆S b are reported for the heating cycle rather than the cooling cycle because of the sharp melting transition. The surface tension was measured with a thin platinum wire of nominal diameter 0.5 mm attached to Wilhelmy Cahn 2000 microbalance. The actual diameter of the wire (0.53 mm) was calibrated using heptane. The error in absolute surface tension measurements is ( 0.5 mN m -1 . The cell temperature was changed in steps of 0.2 K at the rate of 0.1 K/min ...

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Metastable States Observed Due to Phase Transformation Kinetics in Polymers

Cheng¹

2008

Phase Transitions in Polymers

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Observation of Novel Liquid-Crystalline Phase above the Bulk-Melting Temperature

Cited by 22 publications

References 16 publications

Partial Crystallinity in Alkyl Side Chain Polymers Dictates Surface Freezing

Partial Crystallinity in Alkyl Side Chain Polymers Dictates Surface Freezing

Thermodynamic Study of a Novel Surface Ordered Phase above the Bulk Melting Temperature in Alkyl Side Chain Acrylate Polymers

Metastable States Observed Due to Phase Transformation Kinetics in Polymers

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