Poli(akrilamid) hidrojellerinin mikroskopik özellikleri ve makroskopik elastik davranışları arasındaki bağıntı

Correlation between the microscopic properties and macroscopic elastic behavior of poly(acrylamide) hydrogels

Özet

Bu çalışma kapsamında Akrilamid (AAm) monomeri ve N,N'-metilenbisakrilamid (BAAm) çapraz bağlayıcısı kullanılarak farklı başlangıç monomer konsantrasyonlarında ve değişen miktarlarda çapraz bağlayıcı içeren poli(akrilamid) (PAAm) hidrojelleri sentezlenmiştir. Hidrojellerin şişme ve elastisite ölçümleri gerçekleştirilmiştir. Statik ışık saçınımı tekniği ile hidrojellerin inhomojenite derecesi ölçülmüştür. Yapılan ölçümler sonucunda, PAAm hidroj ellerinin mikroskopik özellikleri ile makroskopik elastik davranışları arasında bağıntı olduğu gözlenmiştir. Hidrojellerin sentez sonrası elastisite ölçümlerinden, BAAm' in çapraz bağlanma etkinliği $epsilon$xı yani etkin çapraz bağ oluşturan BAAm oranı hesaplanmış ve sentez sırasında kullanılan başlangıç monomer konsantrasyonu azaldıkça $epsilon$xı' in belirgin bir şekilde azaldığı görülmüştür. Başlangıç monomer konsantrasyonu $C_o$ = 3 w/v% olan jeller için $epsilon_{xl} = 10^{-2} - 10^{-3}$ olarak bulunmuştur. Bu sonuç; sentez sırasında kullanılan BAAm' in 99 ile 99.9 % aralığındaki büyük bir kısmının halka ve çok-katlı çapraz bağlanma reaksiyonları gibi yan reaksiyonlarda ve elastik olarak etkin olmayan bağların oluşumunda harcandığını göstermektedir. Hidrojellerin sentez sonrası ışık saçınımı sonuçlarının Debye-Bueche teorisi ile yorumlanmasından, PAAm hidroj ellerindeki inhomojenitenin boyutunu veren korelasyon uzunluğu $xi$, yaklaşık 10 nm olarak bulunmuştur. Sentez sırasında kullanılan başlangıç monomer konsantrasyonu arttırıldığında, BAAm' in çapraz bağlanma etkinliğinin arttığı ve dolayısıyla PAAm jellerindeki konsantrasyon dalgalanmaları $langleeta^2rangle$ 'nın azaldığı görülmüştür. Jellerin elastise ve ışık saçınımı ölçümlerinden elde edilen sonuçlardan; polimerizasyon reaksiyonu sırasında boşa harcanan çapraz bağlayıcı nioleküllerinin miktarı ile jellerin yapısal inhomojenitesi arasında bağıntı olduğu anlaşılmıştır.

Abstract

Hydrogels have been receiving increasing attention due to their intrinsic scientific interest and their potential clinical and technological applications, in which, precise information on the elastic properties and swelling behavior of gels are required. In recent years, research conducted on the microscopic structure of gels indicates an inhomogeneous crosslink density distribution throughout the gel sample known as the spatial gel inhomogeneity. The fluctuation in the crosslink density of gels is undesirable for the applications because the structural gel inhomogeneity are expected to affect the mechanical, optical and thermal properties of gels, as well as leads to deviations between the predictions of gelation and rubber elasticity theories and experimental data. Thus, understanding and controlling the spatial inhomogeneities of gels has been a challenge in polymer science. It is well known that free-radical crosslinking co-polymerization has been widely used to synthesize the polymer gels. Gelation process and gel growth during copolymerization occur non-randomly due to the conversion and structure dependent reactivities of the functional groups as well as due to the cyclization and multiple crosslinking reaction (Funke et. all., 1998; Okay et. all, 2000). Therefore, polymer gels formed in such a non-ideal picture necessarily include spatial gel inhomogeneity. In contrast to ideal gels with constant network chain length between crosslinks, real gels exhibit a wide distribution of chain lengths between network junction points. Furthermore, in ideal gels, it is assumed that all crosslinker molecules are consumed by effective crosslinking reactions. Since the gel inhomogeneity is closely connected to the spatial concentration fluctuations, scattering methods such as light scattering and small-angle neutron scattering have been employed to investigate the spatial inhomogeneities(Mallam et. all, 1989). The gel inhomogeneity can be manifested by comparing the scattering intensities from gel and from a semi-dilute solution of the polymer at the same concentration. The scattering intensity from gels is always larger than that from polymer solution. The excess scattering over the scattering from polymer solution is related to the degree of inhomogeneity in gels. The spatial inho mogeneity in acrylamide-based gels has been investigated as a function of a number of parameters, such as the crosslink density(Shibayama vd., 1994), the type of the crosslinker(Lindemann vd., 1997), and the ionization degreefMoussaid vd., 1994). The initial monomer concentration used in the gel preparation significantly affects the network structure and the gel properties (Baker et. all, 1994). Decreasing the polymer concentration prior to crosslinking causes the polymer chains to disentangle, so that the network formed in a dilute solution can swell highly when exposed to a good solvent. On the other hand, decreasing polymer concentration increases the probability of cyclization and multiple crosslinking reactions during gel formation process, so that a large fraction of crosslinker molecules are wasted in ineffective crosslinks (Okay et. all, 1995). Since the structural inhomogeneities of gels are mainly dependent on how the gel was formed, it is important to study the effect of formation conditions such as the type of crosslinker and concentrations of crosslinker and monomer. In the present work, several sets of PAAm gels were prepared at various initial monomer concentrations and crosslinker content. The gels were characterized by swelling and elasticity tests as well as by static light scattering measurements at a gel state just after their preparation. A correlation was found between the macroscopic elastic behavior of PAAm gels and the microscopic gel structure. The effective crosslink densities of the gels were determined by the elasticity test. It was found that the crosslinking efficiency of BAAm $epsilon$xi that is the fraction of BAAm forming effective crosslinks decreases as the initial monomer concentration $C_o$ is decreased. At $C_o$ = 3 %, $epsilon_{xl}$ was found to be $10^{-2} - 10^{-3}$, indicating that 99 to 99.9 % of BAAm used in the gel preparation are wasted in elastically ineffective links. Debye-Bueche analysis of the light scattering data showed that, irrespective of the gel synthesis conditions, the correlation length $xi$, that is, the extension of inhomogeneities in the gels is 10 nm. The extent of frozen concentration fluctuations in the gels represented by $langleeta^2rangle$ decreases with increasing crosslinking efficiency of BAAm. The combination of the light scattering and the elasticity data of gels shows a direct correlation between the fraction of wasted crosslinker molecules during gelation reaction and the spatial gel inhomogeneity.