Effect of pH and temperature upon self-assembling process between poly(aspartic acid) and Pluronic F127

doi:10.1016/j.colsurfb.2014.04.023

Colloids and Surfaces B: Biointerfaces

Volume 119, 1 July 2014, Pages 47-54

https://doi.org/10.1016/j.colsurfb.2014.04.023 Get rights and content

Highlights

•
Evaluation of poly(aspartic acid) and Pluronic F127 self-assembling capability.
•
pH and temperature effect on interpolymeric complex formation.
•
Establishing the best ratio for interpolymeric complex formation.

Abstract

The present investigation was made in order to evaluate the capability of self-assembling of the two water soluble polymers, respectively, poly(aspartic acid) and Pluronic F127 into well interpenetrated mixture, and to evidence the connection effects intervened during polymer complex formation to exhibit good stability once formed, as well to understand and correlate the binding strength and the interval between better association domains. The effect of pH and temperature on the interpolymeric complex formation between poly(aspartic acid) and Pluronic F127 was studied by combining rheology with light scattering technique. The solution mixtures between poly(aspartic acid) and Pluronic F127 are Newtonian fluids for all ratios among them. Depending on the polymeric mixture composition and experimental temperature, positive or negative deviations of the experimental values from the additive dependence appear. An interesting behavior was registered around 1/1 wt. ratio between the two polymers, when the hydrodynamic diameter of the interpenetrated polymeric particles decreased suddenly. This allows us to conclude the formation of core–shell micelle structure with poly(aspartic acid) core and Pluronic F127 as shell, performed through strong interactions between polymers. This behavior was sustained by the increase of absolute value of zeta potential owing to the decrease of functional groups number at the surface of micelles.

Graphical abstract

Introduction

Self-assembling is a strategy for materials preparation which can produce highly structured, compositionally defined, multi-component and multifunctional materials from a discrete set of molecular building blocks. Self-assembly has become very useful in chemistry and materials science for diverse applications, in fields ranging across electronic materials, synthetic biology, structural materials, chemical biology, and biomaterials. Thus, through spontaneous self-assembly transitions of the initially disordered molecules, acquired by specific noncovalent interactions, predictable supramolecular structures can be prepared. The forces that direct contribute to molecular self-assembly are weak intermolecular interactions between molecules in solution, which include hydrogen bonding, hydrophobic interactions, Coulombic interactions, π-stacking, and van der Waals forces [1].

During preparation the self-assembling systems are highly dependent on solution conditions such as pH, ionic strength, concentration, temperature, and solvent polarity. The self-assembling processes enable the achievement of materials displaying complex combinations of molecular features. Multifunctional materials can be thus systematically assembled and optimized, even when multiple co-assembling constituents are present, compounds which are also able to be functionalized with ligands or chemical groups either post-assembly or pre-assembly.

Soft condensed matters or biological systems are generally self-associated through weak forces, which are acting together being as well difficult to unravel their relative contributions. For instance, the aggregates formed in water by association between cationic and anionic polymers, or surfactants, become strongly hydrophobic and precipitate in many cases. Also, hydrophobicity results from the hydrogen bonds between a hydrogen donor and a hydrogen acceptor, such as non-dissociated polyacids and polyoxyethylene or polyethoxylated non-ionic surfactants. On the other hand, the polyacid ionization at high pH reduces or even destroys the ability to form hydrogen bonds [2]. Interpolymer complexes are also prepared based on attractive interactions between appropriate macromolecular chains dissolved in a common solvent. These complexes are mainly stabilized via electrostatic interactions in the case of a polyanion/polycation mixture, or through hydrogen bonds between a polyacid (H-bond donor) and a polybase (H-bond acceptor). These complexes have long been known and studied by different techniques, such as potentiometry, excimer fluorescence, rheology and dynamic light scattering [3]. Detailed studies concerning the association between poly(acrylic acid) or poly(methacrylic acid) with poly(ethylene oxide) (PEO) chains have shown that the complexation process depends on the dissociation of the polyacid chains. When the density of carboxyl groups is diminished by raising the pH and neutralizing the polyacid, the complexation diminishes or even vanishes if more than ∼12% of the acid groups are dissociated. This means that a minimal sequence of unbroken bonds is needed to provide a stable complex [4], [5]. During polymer complexes investigations Iliopoulos et al. [6] have distinguished two extreme types of structure depending on polymers concentration and degree of dissociation of a polyacid: the first one is compact and implies a low viscosity of the mixture, and the second one is a highly branched structure, close to a gel, inducing very high increase in viscosity. Thus, the viscosity of a mixture may be several hundred times higher than the sum of viscosities of the two individual polymer solutions.

The present investigation is focused on evaluation of the capability of self-assembling of the two water soluble polymers, respectively, poly(aspartic acid) (PAS) and poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO_n-b-PPO_m-b-PEO_n) triblock copolymer (PL) into well interpenetrated mixture, and to evidence the connection effects intervened during polymer complex formation to exhibit good stability once formed, as well to understand and correlate the binding strength and the interval between the association domains. The complex formed between these two variants of macromolecules is also a system of interest as giving a particular demonstrative example of interaction between a polymer with dissociation capacity and a structure with surfactant ability. Another perspective of interest is the possibility to develop from these structures pH and temperature dual-sensitive interpolymer complexes, even gels. PEO_n-b-PPO_m-b-PEO_n triblock copolymer surfactants, often known by their trade name as Pluronics, are extensively studied and their behavior is quite well understood. They find many applications due to their ability to modify interfacial properties as well as to self-assemble in aqueous solution [7], [8], [9], [10]. It is well known that the Pluronic F127 (Poloxamer 407) forms thermoreversible gels. This characteristic recommends Pluronic F127 to be used as carrier for most routes of drug administration including oral, topical, intranasal, vaginal, rectal, ocular, and parenteral routes [11], [12]. The potential use of Pluronic F127 as artificial skin has been also reported [12]. In recent years, Pluronic F127 has attracted particular interest in the design of dermal and transdermal delivery systems, with a view to promoting, improving or retarding drug permeation through the skin, bearing in mind that for topical delivery systems, accumulation in the skin with minimal permeation is desired, while for systemic delivery, the opposite behavior is preferred [12], [13]. Poly(aspartic acid) (PAS) and its derivatives possess biodegradability, biocompatibility, and relatively low cost of preparation, which are the main requisites for pharmaceutical applications. PAS multifunctional character allows a variety of modifications following simple chemical procedures, and tailor made. PAS derivatives are used as dispersant, antiscalant, or superabsorber, for home detergents, water treatment chemicals, and oil field treatment additives, for a variety of organic and inorganic solids and scales dispersal, in medicines, cosmetics, and food. These capacities make them good candidates for the preparation of drug delivery systems [14], [15]. The paper refers to the effect of pH and temperature upon the formation of interpolymeric complexes between PAS and Pluronic F127, and the investigations were realized through a combined rheological and light scattering study.

Section snippets

Materials

Poly(aspartic acid) was synthesized in our laboratory through a reaction in two steps (schematic presentation in Fig. 1) [16], [17]. First, poly(succinimide) (PSI) as PAS precursor, was prepared by thermal polycondensation of l-aspartic acid (Fluka BioChemika provenience), in dodecane (Fluka Chemika provenience) at 180 °C, for 6–8 h, using o-phosphoric acid (85% analytical reagent, Chemical Co. provenience) as catalyst. The second step was constituted by in situ hydrolysis of PSI in alkaline

The effect of temperature

The evolution of the complex viscosity (η*) as a function of studied polymer mixture composition, respectively, different content of PAS expressed as weight fraction (w₂), determined at different temperature values, is illustrated in Fig. 2.

The polymer mixtures present positive and negative deviations of the complex viscosity values from the additive dependence, behavior which is dependent on the IPC composition as well as the conditions of temperature during experiments [18]. These

Conclusions

The effect of pH and temperature on the interpolymeric complex formation between poly(aspartic acid) and Pluronic F127 was studied by combining rheology with light scattering technique. The mixtures between poly(aspartic acid) and Pluronic F127 are Newtonian fluids for all ratios among them. Depending on the polymeric mixture composition and experimental temperature it appears positive or negative deviations of the experimental values from the additive dependence. These variations reflect the

Acknowledgments

This work was supported by a grant of the Romanian National Authority for Scientific Research, CNCS-UEFISCDI, project number PN-II-ID-PCE-2011–3-0199 (contract 300/2011).

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Effect of pH and temperature upon self-assembling process between poly(aspartic acid) and Pluronic F127

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Materials

The effect of temperature

Conclusions

Acknowledgments

Eur. Polym. J.

Colloids Surf., B: Biointerfaces

Polym. Test.

Colloids Surf., B: Biointerfaces

Int. J. Pharm.

Int. J. Pharm.

J. Colloid Interface Sci.

Int. J. Pharm.

Eur. Phys. J.

Europhys. Lett.

Macromolecules